Only tangentially related, sorry.

I am incredibly slow at computation (such that on every math test I have ever taken, it took me at least 3 times as long as the second slowest person in the class to finish).

I have tutored a fair amount, and my experience is that when someone says "I am incredibly slow at computation", the real issue is their lack of fluency. After you simplified your first thousand of trig expressions, when you look at the expression 1001, you already see most of the answer, all you have to do is write out the steps.

My model of a learner is that a person is a sort of Markov chain, where you try to go from the NO-SKILL state through the LEARNING state into the HAVE-SKILL state, also known as fluency or mastery.

Learning goes like this:

NO SKILL ----- learning rate ----> LEARNING- ---- internalization rate ----> HAVE SKILL

Forgetting goes like this:

HAVE SKILL ---- slow forgetting rate ---> NO SKILL <---- fast forgetting rate ---- LEARNING

People are extremely different in their learning and forgetting rates, which are also individually subject-dependent. Some learn a new skill quickly, others take awhile. Some retain 90% of the skill from one session to the next, others barely 1%. The internalization rate is less variable. As long as you manage to mostly keep in the LEARNING state for some period of time, you eventually get to the HAVE SKILL state.

The slow forgetting rate is well,,, slow for almost everyone, so it takes a long time to forget a well-mastered activity. You can probably do long division still (maybe after 5-10 min ramp-up), even if you haven't done any in a decade and it was a real pain to learn the first time.

The apparent learning and forgetting rates also depend on already having the skills similar to the one you are learning, like when building the jig-saw puzzle.

Anyway, my point is that you have likely misdiagnosed yourself. The symptom "incredibly slow at computation" could be a manifestation of one or more of the following:

• being in the LEARNING state instead of the HAVE SKILL state
• having relatively low learning rate and/or fast forgetting rate for the usual amount/frequency of repetition
• trying to learn the skill in isolation, which slows down learning significantly

If you figure out which of these apply to you, odds are you will no longer consider yourself "slow at computation", but, say, "requiring more frequent repetitions than average to master a new computational skill".

This model is, of course, rather simplified, as everyone appears to have their limits which they hit eventually for an advanced enough skill, but hopefully helpful enough for the initial diagnosis.

To get a more introductory -- but still quite thorough, and more modern -- Bayesian perspective, I recommend John Kruschke's Doing Bayesian Data Analysis. Ignore the silly cover. The book is engagingly written and informative. As a side benefit, it will also teach you R, a very useful language for statistical computing. Definitely worth learning if you are at all interested in data analysis.

Also, you should learn some classical statistics before getting into Bayesian statistics. Jaynes won't really help with that. Kruschke will help a little, but not much. The freely available OpenIntro Statistics textbook is a very good introduction.

I recommend first reading OpenIntro, then Kruschke, then Jaynes.

A while back, I tried reading Jaynes carefully (i.e. working lots of derivations while reading). I'll share my thoughts, but since I stopped after two and a half chapters, YMMV if you read further.

(1) I felt like I was reading a physics textbook. I'm a recovering physics major, and the experience gave me a serious case of Griffiths deja vu. For example, Jaynes does things like:

• Play fast and loose with Taylor series expansions
• Give arguments based on intuition and/or symmetry
• Assume all functions are well-behaved
• Use concepts / notation from calculus that I've completely forgotten (or never learned)

If you've taken university level physics before, then you should feel somewhat at home reading the first few chapters of Jaynes. If not, I would recommend putting in a bit of extra effort to make sure that you understand EVERY step of important arguments / derivations.

(2) After several days of effort, I got to the point where... you could show that if an urn has 3 red balls and 7 black balls, then the probability of drawing a red ball is 3/10. Yay!

Ok, fine, to put it another way, by making a few VERY basic assumptions about reasoning under uncertainty, you can show that the laws of probability are uniquely determined.

If you think this is the coolest revelation ever, then you should definitely read Jaynes. On the other hand, If you'd rather learn how to win at poker, or analyze randomized algorithms, or do calculations about 3d random walks in a cylinder, or something, then Jaynes is probably not the right textbook for you at this time.

So, onto your questions:

What math topics do I already need to understand to prepare myself for this?

Calculus, how to Taylor expand, how to carefully and patiently follow a long argument. I would recommend against going down a deep rabbit hole though (e.g. I would discourage trying to learn "all of multivariable calculus" before starting Jaynes).

Is there a better book to learn probability theory?

It depends; probability theory is a huge topic, and you can attack it from many different angles depending on your goals and interests (e.g. where you want to apply it, whether you're learning it as a prerequisite for another topic). That said, if what you're after is LessWrong / CFAR street cred, then I would probably stick with Jaynes ;-)

Here are some alternatives:

• Start with a problem book, maybe this one
• Go for something that's a bit more math-ey (and less physics-ey)
• Richard Hamming is cool

See this discussion in The Best Textbooks on Every Subject

I agree that the first few chapters of Jaynes are illuminating, haven't tried to read further. Bayesian Data Analysis by Gelman feels much more practical at least for what I personally need (a reference book for statistical techniques).

The general pre-requisites are actually spelled out in the introduction of Jayne's Probability Theory. Emphasis mine.

The following material is addressed to readers who are already familiar with applied mathematics at the advanced undergraduate level or preferably higher; and with some field, such as physics, chemistry, biology, geology, medicine, economics, sociology, engineering, operations research, etc., where inference is needed. A previous acquaintance with probability and statistics is not necessary; indeed, a certain amount of innocence in this area may be desirable, because there will be less to unlearn.