The Theory of Poker by David Sklansky (1989, Paperback)

This is advanced poker theory.
While this book will certainly make already good poker players even better poker players, it won't do anything for beginners or intermediate players. This book addresses a lot of the theoretical underpinnings of the game that are rooted in intermediate maths and advanced game play psychology which will not mean much to people whose poker game isn't "there yet". Before buying this book, you must be honest with yourself about where your game is. You wouldn't expect 3rd Graders to enjoy reading Faust no more than you can expect beginning or intermediate players to really grasp this book.

With that said, if your game is "there"- if you are already a consistently winning online or B&M poker player- this book is one of the few "must reads" in the genre of poker publications.

I made the mistake and bought this book way, way too early in my journey as a poker player. I gave it away, since it meant nothing to me. Now, years later, as a winning player whose game is immeasurably more advanced than it was when I first got this book, I understand and comprehend it's value.