Magriel on Backgammon (PDF)
Backgammon clearly explains how to play good backgammon in a way that no book before or since has quite matched. Having defined the rules and the basic game types, it explains with excellent examples the principles of duplication and diversification, and the strength of the 5-point. It lays out rules for when to play boldly and when to play safe, when to split your back men and when not to split them. Many other basic (and some not so basic) backgammon concepts are explained. The author leads the reader on the journey from complete novice to competent player. The one area of the game which the book covers only lightly is doubling, but even here there are some gems.
This is by far the best introductory book on backgammon. When I first read Magriel, about 15 years ago, I was very weak, and it transformed my game. I always recommend reading Magriel to anyone trying to improve. For players who don't understand the fundamentals of the game, studying this book and trying to apply its concepts over the board invariably leads to tremendous improvement