Robert E. Lee on Leadership : Executive Lessons in Character, Courage, and Vision by H. W. Crocker III
While this book is certainly not a comprehensive biography, this book is indispensable for any one who wants to learn about Lee's character & leadership, especially if you have any position or need of leadership in your life.

Maxims of Robert E. Lee for Young Gentlemen by Richard G. Williams Jr.
This book is a collection of quotes from Lee's writings, letters, etc. especially chosen for young men.

The Recollections & Letters of Robert E. Lee by his son, Cpt. Robert E. Lee
This book gives a good insight into Lee as a civilian, father, educator, & husband. While it is obviously not unbiased, it is a phenomenal peek into Lee's life.