The first book on cigars that I ever read and it's still my favorite....
Nat Sherman's A Passion for Cigars by Joel Sherman
Some of my other favorites are:
The Gorrment Guide to Cigars - by Paul B.K. Garmirian, PhD.
The Gentle Art of Smoking - by Alfred H. Dunhill
The Connoisseur's Book of Cigars - by Zino Davidoff