This one is not just about boas. But it is great for genetics.
Reptile and Amphibian Variants: Colors, Patterns, and Scales.
Other good books taht have not been listed are:
Boas & Pythons and Other Friendly Snakes.
Boas & Pythons Breeding and Care.
The Atlas of Snakes of the World or if you do not want to spend as much The Mini Atlas of Snakes of the World