The Complete Boa Constrictor by Vincent Russo is one of the best books out there. It's expensive, but worth the cost:


http://www.amazon.com/Complete-Boa-C...1981869&sr=8-1

Boas in Captivity by Russo is also pretty good:
http://www.amazon.com/Captivity-Prof...314875-2732449

There's also Red-Tailed Boas: a Complete Guide to Boa Constrictors: http://www.amazon.com/Red-Tailed-Boa...1982023&sr=8-2