The Barker's book was not only the best I've ever read on ball pythons, but the best out there on snakes in general. The detail and dedication to every aspect of ball python care is the level of detail we should strive to achieve with all the species we keep.
The Kevin McCurly book is also a great one for basics, not just in care but in understanding a punnett square and genetics.
Happy to see that my two favorites were the first posted :)
Other books that are worth perusing for an alternate perspective and/or to round out your knowledge:
The european perspective on snake keeping - this was an eye opener for me when I first started working here at LLL. This book is kind of old, and translated from german, but it's a good read:
Ball Pythons: Habitat, Care, and Breeding
An excellent read on keeping your snakes in more than just the bare minimum of caging. For ball pythons, somewhat less relevant, but again this was an eye opener for alternate ways of housing reptiles. Why NOT naturalistic displays? :)
The Art of Keeping Snakes
For the serious keeper, naturally you need a book on illnesses.
Snake Diseases - Preventing and recognizing illness
-Jen