Hank Haney's "Essentials of the Swing". A seven point plan to building a better swing and shaping your shot.
Great book, with a lots of useful simple information to improve your game. 

Golf's Sacred Journey, David Cooke
Incredible read for a powerful golf and spiritual message. Will change your life.

Golf is Not a Game of Perfect, Dr. Bob Rotella
Master of the mental game, a must read also.
 
Little Red Golf Book, Harvey Penick 
Classic book on the game. A must for your library.

How to Play Golf, Sam Snead
Another classic that brings out the "old school" style of the game from years ago. Some great tips on scoring.

Tales From "Q" School, John Feinstein
Loved this book and the characters. Shows how tough it is to make a living playing golf.

USGA Rules of Golf (latest edition)
Of course we listed his one, keep it in your golf bag.