I think one book that is a must have for all guitarist is a book called "The Chord Wheel" written by a man named Jim Fleser. I ran across his book several months back at a library, and ever since then I found myself thinking about his book.
I finally broke down and bought it, and it was the best twelve or so bucks I've ever spent on theory. Bar none this is one the most valuable and essential tools you will come across. And as far as giving one the ability to compose in key, I hate to sound like a billboard for the man, but I have to give it up for Jim, he makes it simple, and that goes a long way in teaching anyone theory.
His web address is www.chordwheel.com
Please let me know if this helps you like it has helped me. I wouldn't recommend anything I haven't tried myself and I'm sure it won't disappoint you.
Best Regards
Cobalt 29