The best book I have seen is:
Dimensioning and Tolerancing
ASME Y14.5M-1994
It is put out by: The American Society of Mechanical Engineers
It's real good at explaining EVERY question that I have had come up about GD&T as well as everyone else's that I have had to deal with.
I obtained my copy from:
CMM Technology Inc.
(1-800-648-6944)