Sklansky's The Theory of Poker has generally been reguarded as a classic since its initial release. Along with Doyle Brunson's Super System and Mike Caro's Book of Poker Tells these three book were the the 1-2-3 knockout combo of poker books. Super System covered many main types of poker specifically, Caro's Book of Tells covered much of the psychology of poker, but Sklansky's Theory of Poker was quite different.
Instead of focusing on one aspect of poker, Sklansky decided to focus on the "big picture". Odds, psychology, information, using these things in combination to make the correct decisions is what poker is all about. And with The Theory of Poker Sklansky wrote a book that attempted to teach you how to do just that. Simply stated, if you make more correct decisions than your opponent, and make less wrong decisions, then in the long wrong, you will win. Luck is the thing that keeps beginners and gamblers coming back to try and "hit it big". And it's what keeps those in the know in the money.
That said, this book is not the easiest read for the uninformed. I admit that I was one of those caught up in Poker big boom a few years ago but I have truly fell in love with it since then and have become a true student of the game. I have gone from dead money to a profitable player - and alot of that thanks goes to many of the books I have read by many great authors. In an attempt to categorize them to help beginners like myself choose what's right for them (in order):
Beginners:
Phil Gordon's The Real Deal - A very easy read to get beginners thinking about the game.
Sklansky's Hold'Em Poker - Not much more complicated than Phil's book and offers more good ideas for the novice for getting started.
Caro's Book of Poker Tells - Pretty straight-forward. Even most beginners should be able to grasp the concept of the tells and the psychological aspects of poker. Just beware of others who've read this book.
Intermediate:
Sklansky's The Theory of Poker - Certainly a must have. Will definitely get you thinking critically about the game.
Doyle Brunson's Super System - A classic, but you won't be able to dominate the tables like Doyle used to. And considering all the poker types it covers it's an invaluable book.
Harrington on Hold'Em vol. 1 and 2 - Even though this applies to mostly tournament play, much of this advice can be used in cash games. But if you play many tournaments (like I do) these are invaluable books and I would even put them ahead of Super System.
Phil Gordon's Little Green Book - A great suppliment to The Real Deal. Offers some great ideas and concepts and a pathway into the mind of one of the better Hold'Em players in the world.
Advanced:
Sklansky's Hold'Em for Advanced Players - Simply the best book on Hold'Em written but a pain in the brain to read if you're not in the know. Definitely work your way up to it.
Poker Essays vol. 1-3 by Mason Malmuth - Much like The Little Green Book these offer some great ideas but is more advanced. I still highly recommend them.
There are no doubt many other great books out there I haven't read. But the fact that these books have helped (and I say helped, because it requires much more than just reading books) turn me from dead money to a profitable player is good enough for me to recommend them. I recommend buying them in the order listed if you're new to Poker or reading them in that order if you've already bought several.
***UPDATE***
It's been more than a year since I wrote this review, and since then there's been a wealth of new poker literature out there.
Among them, Sklansky's No Limit Limit Hold'em: Theory and Practice is the most invaluable. It is akin to this wonderful book, but focuses on No Limit specifically. It belongs in the "super-advanced" section, but it has been the book that has raised my game to the next level.
Also in that advanced category, "The Mathematics of Poker" delves into the complicated math behind the game, and while it is not exactly a practical book, it may introduce the math inclined to an insightful look behind the math that rules the game.
Joe Navarro's "Read em' and Reap" is now the definitive book on poker tells. It was written by a career FBI agent who specializes in reading people. It not only gives you the tells, but goes into the deepest psychologies of what makes people reveal these tells.
Both Phil Gordon's Little Blue Book and Harrington vol. 3 tournament book are invaluable additions to each's library. Harrington vol. 3 especially for its breakdown of famous hands, and the thinking behind them. Weighing the Odds in Hold'em Poker is another superb Limit book, and has become perhaps my biggest aid when playing Limit. Doyle's Super System 2 is a valuable addition to vol. 1 if you wish to read up on various other games besides Hold'em.
I wish the best of luck to everyone at the tables - there's still enough fish to go around!