RGP celebrity Will Hyde, under the pen name Justin Case, and with the help of Steve Jacobs and a bit of computer hardware and software, has put together an impressive collection of tables of hold'em percentages, including winning rates, performance analyses, and incoming rates for all possible starting hands at tables of different sizes. Their numbers are derived from Monte Carlo simulations, in which each possible starting hand was run against random opposition 500,000 times.
If you want to make use of these numbers, it's important to keep in mind that the win rates are only applicable if no one ever folds. That is, instead of getting into the hairy problem of simulating how your opponents might play, Hyde and Jacobs just tell you how often particular hands will end up being the best hand at the table. Since real players often do fold, information like this has to be used carefully, although it can be a good starting point for some analyses, especially in tournament and big-bet poker.
A few technical details are worth noting. First, with modern computers and fast algorithms, it's possible to calculate probabilities for some of these numbers (e.g., how often each hand will end up best against a single opponent with random cards). That is, while large Monte Carlo methods (running a large number of trials) yield accurate results, we can sometimes do a hair better. Second, Case and Jacobs counted tied and non-tied wins identically, which means the numbers are not strictly accurate for certain kinds of equity calculations. Both of these issues are miniscule, however.
The contents of the book really speak for themselves. If you want these tables, and can't or don't feel like generating them yourself, this is probably a good buy. The rest of the book's contents are really beside the point, but it does contain some material on how some probabilities are calculated, and a Q and A chapter with the ins and outs of various situations described in percentages. As a bonus, you also get Will's Lowball Book, supposedly the final word on draw lowball. I don't play lowball, so I won't comment on it, except to say that the writing is lively.