Almost all experts agree that The Theory of Poker is the best book out there for serious players looking to improve their game. It includes basic strategy, answers common questions, and is "the book" for poker players. Every professional and semi-professional player has read this book, and it should be one of your first reads if you're looking to improve your game.
Author David Sklansky has won three World Series of Poker bracelets, attended the Wharton School of Business at the University of Pennsylvania, and has won over a million dollars playing poker. He is an expert on the game in every meaning of the word.
Harrington on Hold'em is the most-purchased poker book ever. Dan Harrington is considered one of the greatest players of all time, and his three-volume book details some of the best poker theory and strategy. Volume one focuses on basic strategy, volume two on heads-up (one v one) poker, and volume three on sample hands. Volume three also includes a workbook for players to improve their own game.
Winning Low Limit Hold'em, by Lee Jones, used to be the book that all poker players read, from professionals to beginners. It is still a great read for people just starting out in the game of Texas Hold'em, and it's easy-to-understand chapters provide an excellent starting point for anyone.
Written by the same man who wrote The Theory of Poker, this book is excellent for the intermediate or advanced player looking to take their game to the next level. This was the first book written exclusively for Texas Hold'em, and includes expert strategies and hand analysis.