The Theory of Poker - David Sklansky
As I'm a big fan of discussion, and also of reading books, I figured I could try and jumpstart a discussion on books. And since everything has to start somewhere, I thought Sklansky's "Theory of Poker" to be a decent point of origin.
As the book doesn't present any specific strategies on poker ("play these starting hands", etc), but instead focuses on the very core of the game, it's hard to find good angles for discussion on this book in particular. However, I'll offer this:
I keep seeing people advicing beginners to stay away from this book ("but it's a must-read for experts"), because of the complex nature of its contents.
I disagree, with a disclaimer.
If by "beginner", you mean someone who has never played poker, or maybe only a few hands of 5-card-draw when they were 12, with their dad, then yes, that person would do well to start elsewhere. But if they know the basic rules of poker, this is a good read for anyone.
The problem with this book isn't that it's "difficult" in what it explains, it's just not written in a way that's made for casual reading. But for anyone who's gone to college or read any text-book, it's hardly more complicated than the norm. In fact, the actual math in it is not hard at all. You have to stay focused when you read it, and you need to think about some sections, but I disagree with needing lots of experience before tackling it.
There is one good reason why someone may want to hold off on reading this book, however, but it has nothing to do with the difficulty of understanding what it teaches: For sheer profitability, this book is not going to pay off as much as some other books, early on in your career as a poker player. If you're struggling to break even, and what you really want is something to help you keep your bankroll, then get some other book first, which includes some basic strategies on how and what to play for the variant of poker you're interested in. But if you're looking to build a solid foundation which will help you benefit even more from the other books you'll read, then there's no reason to hold off on buying The Theory of Poker, in my opinion. After you've read it, you'll want to from time to time go back and re-read sections or the whole book. And if you really want to do your homework, you'd do well - while reading a suggested strategy by a different author - to cross check Sklansky's reasoning to see how it fits in. But don't let people scare you into thinking it's too much to muster.