As Bill James is one of the people whose works helped me recover my interest in baseball, I would like to make some small corrections to hashbrownie's writeup in the interest of truth - and paradoxically, adding some mystery and humanity to the image of this sabermetrics demigod.
First of all, the man is not a statistician, something he's been at great pains to point out in many of his works. It's an understandable mistake, the man works with statistics, so he must be a statistician, right? Er, no. Bill James is somebody who chose to study baseball, and he became famous for looking at the numbers which track what is felt to be important in baseball, and trying to figure out what these numbers were really telling us, as opposed to what the coaches and managers and sportswriters and players said the numbers were telling us. Understandably, this pissed off a lot of those people, and in the 1988 Baseball Abstract, James wrote "Breaking The Wand", an essay in which he bemoaned the explosion of new statistics that had erupted forth from the new field of sabermetrics. He felt that many of those numbers were clouding the search for the truth and being used as ends in and of themselves, and he didn't want to contribute to that clouding any more.
This brings me to the second thing about Bill James that needs to be pointed out: he is a terribly humble man, very conscious of his ignorance. It would be very easy for someone who has fundamentally changed the way most people look at baseball to have an ego of Olympian proportions, but instead James presents as a self-effacing mensch who has problems believing sometimes that he's climbed the ladder from night watchman at the pork & beans plant to best-selling author and advisor to one of the legendary Major League teams.
I think it's this humility, this sense of his own ignorance of so many things, that makes him such a great writer. Bill James isn't trying to impress you with how smart or how learned he is; he's more like your buddy who is a total geek about baseball and keeps coming up and saying, "Hey, did you know X? Isn't that cool?" That's part of the reason my two favorite Bill James books don't have a lot of sabermetrics going on in them. This Time Let's Not Eat The Bones is a fine collection of essays, anecdotes, smart remarks and serious writing about baseball teams, baseball players, owners, managers, front office people and other stuff without numbers, because sometimes the numbers just get in the way. Likewise, The Politics of Glory is mostly history and a very serious inquiry into why the players in the Hall of Fame are there and how they got there. It ought to be mandatory reading for anyone who wants to be taken seriously in a discussion of the National Pastime, but that would be too much to ask.
Is it too much to say that Bill James is to baseball what Robert Heinlein is to science fiction? Maybe, and maybe not. I do know, though, that if you want to learn about baseball and its history, you could do a lot worse than pick up a book by Bill James and start reading.