Karen Pryor has a couple - "Clicker Training Your Dog" is a booklet that comes with 2 clickers, which covers the basic theory behind clicker training and a few exercises. She also has "Don't Shoot the Dog" which is more in-depth.
There is a new one, "Clicking With Your Dog - A Step-by-step Approach" (or something like that) I think by Peggy Tillman, I've heard it is great but haven't gotten it yet.
Morgan Spector has "Clicker Training for Obedience" which is more geared toward competition, but also gets heavily into theory.
Shirley Chong has "A Clicker Cookbook" which I am becoming very fond of, it is a step-by-step booklet with a separate training log to track your progress. It also provides goals for each step, so that you know when it is time to move ahead.
All the books except Shirley's are available at
www.dogwise.com or
www.sitstay.com Shirley's are available at
www.shirleychong.com Morgan's and Karen's are also available at
www.sunshinebooks.com or
www.clickertraining.com