David Sklansky's Theory of Poker
Sklansky's Theory of Poker is not an easy read, but it is one the fundamental books of modern poker
Lee Jones' Winning Low Limit Holdem
Jesus doesn't want you to read this book!
DD Tournament Poker
DD Tournament Poker is a good way of practising for multitable tournaments.
Ed Miller's Small Stakes Holdem
This book is the graduate course in beating low limit games.
Doyle Brunson's Super/System 2
Doyle Brunson (and friends) update the Poker Bible (Super/System I)
David Sklansky's Holdem for Advanced Players
Holdem for Advanced players by David Sklansky is one of the most important books about the game.