You really don't need a Boxer-specific training book; general training advice applies to Boxers, too.
The Culture Clash is a good one for any dog owner's bookshelf, by Jean Donaldson. I also like Clicking with Your Dog by Peggy Tillman, and a good companion for that is Click for Joy by Melissa Alexander. (Melissa's book, I'm fairly certain, address the issue of fading the lure - the treat. Peggy's may, as well. It's a common problem for new trainers (and some not-so-new ones!).)