The Theory of Poker is a book from 2 + 2 owner, author, and long time poker player David Sklansky.
Within the poker and gambling community Sklansky's writings on theoretic aspects are held in the highest regard. He is probably the only person who could write a book called The Theory of Poker that would actually stand up to its name, and in this case it most certainly does.
While most of the examples in the book are based on hold'em, the actual content is not specific to any one type of poker. It promotes a basic as well as deep level understanding of concepts such as long term expectation, antes, pot odds, effective odds, implied odds, reverse implied odds, slowplaying, bluffing, getting "free cards", semi-bluffing, defending against all these strategies, adjusting playing style, taking advantage of position, playing heads up, and reading hands, along with the titular theory of poker itself. Fitting all this in under 300 pages is an admirable achievement indeed.
The actual fundamental theory of poker itself states that if you could see your opponents cards and you would change the way you play based on that, you lose in the long run. Likewise, if they could see your cards and they'd change the way they played against you, you win in the long run. An abstract concept for sure, but it makes sense.
It is not a difficult book to read, and it makes sense. It's very well written, and it's appropriate for all skill levels. Beginners, intermediate, and professional players should own and read a copy of this book regularly. Every time you read it you will get something new out of it, for sure. It is a must-own title for anyone serious about the game at any level.