Two pamphlet-length works here: the first, Speculation as a Fine Art, is a foundational old-school text for investors, cited repeatedly in Reminiscences of a Stock Operator.
The second, Thoughts on Life, contains short chapters of quotes and sayings in the style of Baltasar Gracian or Epictetus (see The Golden Sayings of Epictetus, a wonderful book, and it's free and in the public domain.).
Recommend ingesting the first and skimming the second. As with most books of sayings, there's gold in there if you dig for it, and in the notes section below I copy-pasted some of the quotes that grabbed me. Maybe you can even skip skimming Thoughts on Life and instead skim my notes!
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As for Speculation as a Fine Art: it talks through some blunt ideas, some of which investors today may not want to hear. See for example Watts' discussion of the personality it takes to be a competent speculator: an idiosyncratic mix of courage and indifference, plus the ability to know what the average investor is thinking and doing... so you can literally do the opposite. That last part sounds harsh and I don't want it to be true, but yet it's still true.
It also contains bedrock reminders that we "know" but don't always practice when investing, like cutting your losses and letting our profits run (we usually do the reverse, selling winners after a small gain and hoping our losing stocks will come back); or failing to think through proper position sizing; or failing to consider how "future you" will think about a stock once it gets cut in half (usually we idealize how cool and unpanicked we're going to be, and we're usually wrong). Watts condenses these last two ideas in his famous and pithy quote "Sell down to the sleeping point."
If you'd like to not get rudely separated from your money, this very short work is a profitable read.
Notes:
What Is Speculation?
3 Definition of terms; is there any difference between gambling and speculation? "All business is more or less speculation. The term speculation, however, is commonly restricted to business of exceptional uncertainty." On the qualities "essential" to the speculator: self-reliance, judgment, courage, prudence, pliability.
4 "In speculation there is value in Mirabeau's dictum: 'Be bold, still be bold; always be bold.'" On the need to think and act promptly. On the ability to change one's opinion.
5 On having all these qualities in balance. "A deficiency or an overplus of one quality will destroy the effectiveness of all."
Laws Absolute
6 Never Overtrade: never take a position larger than your capital justifies because it "unnerves the operator, and his judgment becomes worthless."
6 Never "Double up": what Watts means here is if you're long don't sell out and then go short; he's referring to reversing a position the other way. "The change in the original position should have been made moderately, cautiously, thus keeping the judgment clear and preserving the balance of the mind."
6 "Run Quickly," Or Not at All: act promptly at the first approach of danger, or if late to see the danger, at the least close out part of the position.
6 When Doubtful, Reduce the Amount of the Interest: "sell down to a sleeping point."
Rules Conditional
7 It is better to average up than to average down; the author notes that this is contrary to the commonly-held opinion, the he cites that averaging down may work four out of five times, but on the fifth time in a permanently declining market the operator loses his head with a loss so great as to bring ruin. Better to buy at first moderately and add to the line as the market advances. [I tend to do the opposite of this and generally average down, but usually my position sizes are way less than these guys would normally carry, and I make sure I can afford to wait years if need be for an investment to work out (see Mohnish Pabrai's dictum that you cannot sell any investment before three years have passed, which causes you to think very carefully about what you invest in!). But once in a while one of these does go down, and down, and down, and right to zero on me.]
7 "To 'buy down' requires a long purse and a strong nerve, and ruin often overtakes those who have both nerve and money. The stronger the nerve the more probability of staying too long. There is, however, a class of successful operators who "buy down" and hold on. They deal in relatively small amounts [Heh, yep!]. Entering the market prudently with the determination of holding on for a long period, they are not disturbed by its fluctuations. They are men of good judgment, who buy in times of depression to hold for a general revival of business--an investing rather than a speculating class." [Note the distinction here.]
8 "The rule is to stop losses and let profits run... Not to have the courage to accept a loss, and to be too eager to take a profit, is fatal. It is the ruin of many.
8 Public opinion is not to be ignored: The rule is, to act cautiously with public opinion; against it, boldly... Every speculator knows the danger of too much 'company.'"
8 Quiet, weak markets are good markets to sell: "But when a market has gone through the stages of quiet and weak to active and declining, then on to semi-panic or panic, it should be bought freely. When vice versa, a quiet and firm market develops into activity and strength, then into excitement, it should be sold with great confidence." [Again, know what "regular people" think and do the opposite.]
8-9 In forming an opinion of the market, the element of chance ought to not be omitted: On accidents that destroy or modify the best calculation. Also on statistics being valuable but subordinate to a comprehensive view of the whole situation. "Those who confine themselves too closely to statistics are poor guides. 'There is nothing,' said Canning, 'so fallacious as facts, except figures.'"
9 When in doubt do nothing. Don't enter the market on half convictions; wait till the convictions are fully matured. "A reserve force should therefore be maintained and kept for supreme movements." [You always, always have to keep a large cash reserve just in case.]
9 "It may be thought that the carrying out of these rules is difficult. As we said in the outset, the gifted man only can apply them. To the artist alone are the rules of his art valuable."
Book 2: Thoughts on Life
Life
[This rest of the book contains quotes, apothegms and dictums about various life domains, including investing; it's in the style of Baltazar Gracián. I've written down some of the more notable ones and added commentaries where I have them.]
11 Compensations do not always compensate. [Recognize there's always black swan risk that you cannot protect yourself from using typical methods of protection]
A danger known is half overcome.
A fault recognized is half conquered.
Fools try to prove that they are right. Wise men try to find when they are wrong. [I keep a variation of this quote at the top of my daily journal:
Wise men don't need to prove their point;
men who need to prove their point aren't wise.
Follow the vague and intangible, and it will become definite and tangible.
12 All see; few observe, fewer still compare.
Destruction must often proceed construction, but most men stop at the former. [Sometimes you really need to have your face ripped off in order to have some of your most successful investments. Also I think you can expand this quote to the civilizational level: Fourth Turnings are necessary.]
The dangerous classes--enthusiasts and fools.
13 Catch thoughts on the fly, for there is no rebound. [Carry around a little notebook to capture them, or use a file on your phone. The best ideas come when you're not expecting them and they often vanish forever if you don't capture them.]
To conquer fate, advance to meet it.
Man's enemies are legion. Ofttimes he himself is his only friend.
Nerve is nerves controlled. [Emotional continence is an increasingly rare superpower these days. Make sure you have a lot of it.]
14 Many lean, few lift.
Look before you leap, but not when you leap.
15 Health is equilibrium.
The man who conforms never transforms.
Respect your limitations; your limitations will not respect you.
Man fears the unknown, despises the uncomprehended. [A sort of corollary here to the midwit problem would be: don't be a midwit and despise things you don't (at first) comprehend.]
16 Put on the brakes in middle life, or the momentum of youth will destroy you. [It's interesting to think of this saying in the cognitive work that I do now that I'm in my 50s: I cannot learn the way I did in my 30s, I had a much faster chip back then, I could learn more quickly, etc., but now I have more mental models and more experience, and so I can sometimes fit new things into an existing mental matrix by thinking by analogy or by heuristic.]
Vehemence is a sign of weakness; quietness is a sign of strength. [Adjacent: don't be vehement, ever, if you want to persuade anybody of anything. You'll just stiffen the other person's mental resistance.]
Among crazy people, a sane man is thought a lunatic.
Great wealth is a misfortune; it attracts parasites and repels friends.
17 Nothing so impressive as simplicity.
The world is full of commonplace people.
Better the vagaries of eccentricity than commonplace dullness.
18 The test of a man is what he is in an emergency.
To separate the essential from the non-essential is the mark of a superior mind.
In his secret heart, every man thinks the universe is especially hard on him. [Haha yep!]
Genius consists of seeing instantly the vital point.
The more points of view, the better the point of view.
19 Don't batter down the door; pick the lock. [Adjacent: Nine Terrible Ways to Make Choices]
It is shadow that makes the difference. [I'm not quite sure what this saying means! I'm including it here to help me roll it over in my mind until I come up with a way to read it.]
Don't fool with nature; she strikes back. [I wish the allopathic medicine industry would read this quote and internalize it; since it won't, we have to internalize it ourselves to navigate the allopathic/pharmaceutical industrial complex.]
Break antagonism with a joke. [Holy cow yes.]
Recognize a fault, but don't dwell on it.
Enthusiasm is a poor guide, but a good companion.
Habit is the tyrant of man; inheritance, the tyrant of the race.
20 All movements are in waves--in politics, and business, in the atmosphere, and spirit. Rest with descending wave; mount with the ascending wave.
At thirty, most men are in a condition of arrested development. A few grow until their bodies fall into the grave.
21 Wealth is a means of refinement; but having done its work it ceases to aid, and retards true refinement.
In youth, a man forges the chains that bind him in old age.
The voice may speak false, but the eye always tells. [See the look of terror in the eyes of former CDC director Mandy Cohen in this post. The eye always tells.]
22 Sickness develops a man inwardly, health outwardly. To have the benefits of both, man must have been sick and become well.
The man who rides a high horse forgets how things look to people on foot.
The realist makes the mistake of exposing everything.
23 It isn't necessary to go through the kitchen to reach the parlor. [Take the path of least resistance, pick the lock, don't batter down the door, etc.]
"Misery loves company," but "company" doesn't reciprocate.
A strong personality arouses antagonisms.
24 Money covers a multitude of sins.
Capacity for suffering is in exact proportion to capacity for enjoyment.
All men travel in circles. A few increase the diameter of the circle.
Angels abroad are often demons at home. [Reminds me of Henry Kissinger's (revolting) heuristic "instability abroad produces stability at home."]
Administer correction with a joke, and it will go down like a sugar coated pill.
25 Tools to those who can use them, words to those who can understand them. [Hmmm: "From each according to his abiiity..."]
In the presence of some people we wither like sensitive plants; and the presence of others, we expand like flowers.
Wealth is at first a means and then a barrier to refinement.
26 Wise men sometimes say foolish things. Fool sometimes say wise things.
Destroy the illusions and there is not much left of life.
Many men have the "courage of their opinions," few the courage to abandon their opinions.
Some people think; others think they think.
At twenty, and again at sixty, a man "knows it all." [I feel like he could be talking about the Boomers here.]
27 Great bodies move slowly. Great events move rapidly. [See also: preference cascades.]
The unpardonable sin--not to make money.
The sorrows of youth are acute; of age bitter.
Stagnation is damnation. Circulation is salvation.
To have made one's self ridiculous, and not to mourn over it, is a supreme test of virtue.
Blame, when praise is deserved, exasperates; praise, when blame is deserved, humiliates.
28 If you wait until you see clearly, you will never act; if you wait for a pure motive, you will never move. [Making this relevant to investing: if you have an obvious reason to buy, don't. It's too obvious and too late.]
A wise man laughs at his own follies. [See also "In Search of the Missing Ox" from Katsuki Sekida's Zen Training: Methods and Philosophy, and the tenth stage, "In town with helping hands": "He mingles with the world. He goes barefoot. He cares nothing about how he dresses. All this symbolizes his mental nakedness. His only thought is to bring joy to others."]
To be sincere with others is easy; with one's self, difficulty.
Never explain. Let your life be the explanation.
Most men absorb the atmosphere they are in. A few create their own atmosphere.
29 Do you wish sympathy? Don't seek it.
Patience is sustained courage.
A fool often condescends to a fool.
30 In life, as in a game of cards, you must sit out bad luck. [You have to make it through, you have to figure out a way to survive the downturns.]
Man seldom wishes advice; he wishes to be confirmed in his own opinion.
Advice is cheap. That is the reason there is so much of it.
Tyranny is the vice of a brutal man; submission to it, the vice of a timid man.
Wisdom consists of seeing many things and concentrating on one thing.
31 We cannot be just and hold the scales ourselves. [Whenever you see a situation where somebody grades his own test--examples like Enron, government inflation reports, etc.--be very wary.]
Keep your voice down and you will keep your temper down.
The wound that injustice makes goes deeper and lasts longer than any other.
Teach by indirection rather than bi-direction; by suggestion, rather than instruction. [People usually can't handle the answer given to them bluntly and directly. Or they respond with reactance or a stiffening defense of their existing opinion.]
Never do today what you can as well do tomorrow. The wisdom of tomorrow is better than that of today. [This is an interesting and counterintuitive saying, and it's often right.]
32 If you want to go anywhere, start. If you want to do anything, begin.
A man who has made a mistake suffers enough. Don't "throw it up" to him.
Two kinds of wisdom--to persist in things worth doing; to abandon things not worth doing.
Courage consists in doing the thing you are afraid to do. [I'd also include in here "things you are egoically afraid to do."]
To advise is to claim superiority and is resented, unless the superiority is admitted.
A good man thinks the motives of others are as pure as his own. A bad man thinks the motive of others are as bad as his own. Life often corrects the mistakes of the former, seldom those of the latter.
33 Life is a dream. Some men know they are dreaming; others think they are awake. [Heavy.]
All things demand an outlet.
Two kinds of vision--to see things as we see them; to see things as others see them. [One of the worst liabilities you can possibly have is solipsism.]
Men wish to die when they should live; wish to live when they should die.
When in adversity, don't speak of it; you will make others unhappy.
When in prosperity, don't speak of it; you will also make others unhappy. [Heh. These last two are good ones to remember: nobody wants to hear about all the terrible things that are happening to you, and nobody!!1!! wants to hear how awesome you're doing.]
34 Some men are alive after they are dead; others are dead while still alive.
Argument is an effort of each man to force his idea on the other semicolon discussion is an effort to gain knowledge. The wise man declines argument, invites discussion.
Do you want a secret kept? Don't tell it. [Less obvious than it appears.]
A true view can only be gained by having been in a thing and having come out of it. [E.g.: adversity, illness; see also the quote on page 22 above about being sick, coming out of it and becoming well.]
Two kinds of truth--abstract and practical. Practical truth is as much of the abstract truth as can be applied at any given time.
Man begins in simplicity, advances to complexity, returns to simplicity. The last is complexity reduced to the fewer terms. Society follows the same course. It is now in the complex stage.
35 To know when to begin is easy; when to stop, difficult. [As much as I try to obey the phrase "always leave 'em wanting more" I more often fail than succeed. I always want just one more wave...]
Vanity struggles long and dies hard.
Bragging is an expensive luxury. Better not indulge in it. [Bragging is even worse than complaining.]
36 Old young men are invariably wicked; young old men, universally good. [Interesting!]
If you work yourself down, sleep yourself up.
Imagination makes cowards of us all.
The rich have few friends; the poor, no enemies.
Business
37 Take short views. [This is exactly what a stock operator would say.]
If you shoot at a near mark, even if your hand shakes you will hit it. If you shoot at a distant mark, and your hand vibrates slightly, you will miss it. [Flipping this insight for a moment: Mohnish Pabrai made an interesting statement once about aiming to become a billionaire: he argued that even if he missed the mark by 50% or even 90% (implying net worth of 100m) he would still do all right.]
When danger threatens, don't stand like a sheep; run like a deer.
Make your theories fit your facts, not your facts your theories. [Use a "falsifying" mindset with your theories: look for facts that contradict your views. With investing, always try to steelman the contra-argument.]
Don't storm the fortress of fortune; lay siege to it. [Plan it out, take your time, use good strategy and tactics, etc.]
Opportunities are always there, but the opportunist is lacking.
Stubborn men don't live long--financially. [Holy cow is this ever true.]
38 Trust your impulses; they are often a higher reason. [Yep.]
The foolishness of the many is the opportunity of the few.
The man who thinks right will gain much.
The man who thinks quick and right has the world in a sling.
The more hindsight the better foresight. [This is why it's critical to read and know your monetary history and your stock market history!]
It is always the unlucky man who believes in luck.
Not the knowledge of facts, but the interpretation of facts differentiates the successful from the unsuccessful man.
Reject green wood, raw men, and new enterprises; wait until they are seasoned. [Lindy Effect]
Business is a kaleidoscope, continually changing and forming new combinations. Take a fresh look every day.
39 "A sure thing" is a dangerous thing.
Seeing things too soon is as bad as seeing things too late.
Act on the temporary; the temporary may become the permanent.
Luck exists. But the able man acts so as to minimize bad luck and augment good luck. [Note that @BowtiedBull frequently says to avoid unlucky people, under the idea that there's some second-order effect involved.]
Take care of the losses; the profits will take care of themselves.
Better to make new money than to go to law and collect an old debt. [Corollary: never lend money to friends.]
40 Beware of the unfortunate man; flee from the enthusiastic man.
Better capital in a man's head then capital in a bank.
Look after the principal; the interest will look after itself.
In the business world, as in the physical and moral world, plasticity is life, rigidity is death.
To be "too greedy" is as bad in business as in morals.
41 Learn principles. Facts will then fall into their relations and connections. [As I get older and have the benefit of more experience under my belt (in both life and investing), I find I am guided more and more by my heuristics and general rules.]
Quick decisions are the best decisions.
When you begin to doubt, begin to get out.
If a speculation keeps you awake at night, sell down to the sleeping point.
Men
42 Little men talk of people; great men, of things.
The self-important man is seldom important to others.
Money adds nothing to an extraordinary man, but it is the saving grace of an ordinary man.
The able man compares himself with the known and is proud. The great man compares himself with the unknown and is humble.
An able man disdains the wisdom of other men; a great man uses the wisdom of other men.
43 The little man demands to be understood; the great man is content to be misunderstood. [See above: "men who need to prove their point aren't wise."]
44 Men who go straight to the point either see very little or see very much.
A real man has no appearances to keep up.
Strong men are silent. When a strong man begins to talk, he is losing his power.
Society
46 "Make believe" is a game society plays as well as children.
Second-class people--those just below us.
Society people order their clothes, but get their opinions "ready made." The world is not deceived; it distinguishes between that which is "put on" from that which grows on. [This is a good one, a rich quote.]
47 Men wear masks and the world takes them seriously; when a man shows his real face, the world laughs.
Nondescripts--"nice people." [While in general it is best to be nice, at certain times it can be a large liability to be nice.]
Man seeks society because he can't endure his own companionship.
Society is one organism.
Language
48 Words are counters in the game of life. Use them carefully; they must be redeemed.
The language of sorrow is tears; the language of despair, silence.
Thoughts are vitalized blood.
We can say things we can't write; write things we can't say.
To know when to speak is rare; when to be silent, rarer still.
49 There is a language of silence, a language of diplomacy, a language of commerce, a language of spirit. To understand a man, you must know the language spoken.
A word in time saves nine.