If, for many, the poker bible is Doyle BRUNSON's "Super System", for me it's David SKLANSKY's "Theory Of Poker" (TOP).
Published in 1979, as Doyle had already captured two WSOP main event titles, Super System probably came as a bombshell as it was the first poker book of quality, with a bunch of specialists telling you how to play their favorite variant (Doyle reserved for himself the infamous NLHE part).
A few years later (1987),David SKLANSKY, who had contributed to Super System, wrote TOP.
The 2 books are fundamentally different. One is presented by a world renowned and egomaniac player, the other is the work of a mathematician who tried for years to dissect every mechanism of poker.
In the end, no doubt Doyle is a more skilled player than Sklansky, mainly as concerns no limit games. But don't expect Super System to reveal all the "mysteries" of poker. You will find a lot of bragging from Doyle, not that much technique in the NLHE section, but, notwithstanding, the valuable description of the way to play a few key hands in NLHE (big pairs, gutshot straight draws) and the infamous Doyle's aggressive way of playing.
In contrast, SKLANSKY is more like a teacher who patiently shows you all the concepts you will have to understand when, seated at the poker table, you will have to take a decision. For me, that makes TOP much more valuable than Super System.
In 25 chapters, each one about 10 pages long, TOP will teach you almost all tools a professional player may use to decide the way he's gonna play a hand. It begins with a few chapters introducing the basic mathematical tools you will need to analyse a hand (like the expected value of a hand, pot odds etc). At that point, you will start to understand that the main question is not "have i won that pot?" but "have i made money on the long run with the way i played that hand?". Don't be afraid, everything about maths in this book is rather basic, you don't need any special level of education in maths.
Let's make it simple, here are the 25 chapters.

Chapter		Comment
1.	Beyond Beginning Poker	...
2.	Expectation and Hourly Rate	Definition of the expected value of a hand, which is of paramount importance to learn how to analyse a hand.
3.	The Fundamental Theorem Of Poker	This theorem says "Every time you play a hand differently from the way you would have played it if you could see all your opponent's cards, they gain; and every time you play your hand the same way you would have played it if you could see all their cards, they lose. Conversely, every time opponents play their hands differently from the way they would have if they could see all your cards, you gain, and every time they play their hands the same way they would have played if they could see all your cards, you lose".
4.	The Ante Structure	...
5.	Pot Odds	After the expected value, another mathematical tool that any decent player should know.
6.	Effective Odds	...
7.	Implied Odds and Reverse Implied Odds	...
8.	The Value Of Deception	...
9.	Win the Big Pots Right Away	...
10.	The Free Card	When can you afford to give a free card to your opponents?
11.	The Semi Bluff	Semi bluff is a very powerful weapon from which it's difficult to defend oneself.
12.	Defense Against the Semi Bluff	...
13.	Raising	...
14.	Check Raising	...
15.	Slowplaying	...
16.	Loose and Tight Play	...
17.	Position	...
18.	Bluffing	...
19.	Game Theory and Bluffing	When your opponents are very tough, better than you, the game theory provides a play that limits the edge they might have on you. Game theory can help in many other situations. A few examples given by SKLANSKY are really edifying as they show that sometimes, with a lesser starting hand than your opponent, you can find a way to play it that makes it a long run winner, whatever your opponent will decide to do!
20.	Inducing and Stopping Bluffs	...
21.	Heads-Up On the End	Regrettably, this chapter conclusions are mostly usable for limit games. Using them in big bet poker could be catastrophic.
22.	Reading Hands	What should be your reasoning to put your opponent on a hand/a range of hands?
23.	The Psychology of Poker	...
24.	Analysis at the Table	This chapter will describe a quick and efficient way of reasonning that only requires a reasonable amount of time when, in the heat of the action, you put your opponent on a range of hands (not on a unique hand).
25.	Evaluating the Game	How can you evaluate a game and decide if it's worth to enter it?
Appendix A.	Rules of Play	...
Appendix B.	Glossary of Poker Terms	...

Franly, there is almost everything in this book. After reading (and understanding) it, you will have a series of tools making you able to make the best play in any situation. So, the cost of the book is modest compared to the amount of money you will win/avoid to lose in the future.
The snag is TOP won't often tell you directly how to play some situations (few hands are given as examples and the majority of them are for stud, razz or limit hold'em, very few NLHE hands then). It tells you how to think, which concepts you have to take into account, but the decision is yours in the end. It will take a considerable amount of time to be able to use all these tools in real poker situations. But it's something you have to know: good players don't know by heart how to play every hand, on many occasions, they have to build a reasoning, as explained in TOP, to come to a decision.
One can regret too that the book deals too much with limit games, a few suggested plays can't be applied directly to big bet poker (like the "Heads-Up On The End" chapter). Now no limit games are much more popular (in France no limit has always been much more popular than no limit), so maybe it would be time for SKLANSKY to update a few things in TOP.
The book being rather theoretical and sometimes rather dense in information, it's not easy to read it quickly. That may be an issue for some readers but be sure that it's worth every second you need to understand it. Give yourself reasonable goals: for instance read a chapter every week. Play a few hands to try to put what you just learnt into practice. Nothing obliges you to read it fast, everything that matters is that you understand everything you read.
The book dealing with poker in general, maybe you will need an additional book, more specialized on a single variant. That's something i strongly suggest if you are still a beginner, as TOP won't tell you which starting hands you should play/raise for example.
In summary, it's an abolute must-read, but i wouldn't suggest it as a first read if you are a beginner. Learn the rules, play a few thousands/tens of thousands of hands, maybe buy a book specialized with your favorite game, and then buy TOP.