I'm a fan of Julian Sprung's Corals: A Quick Reference Guide.
When I first started keeping corals, I'd actually bring the book shopping with me so I could look up what I was considering buying. One one page, I'd get the scientific name, common names, where it's from, what light requirements, placement and feeding as well as some text about each colony. I found it to be really helpful.
I used to keep a copy in the shop too, and referred to it often while helping customers.
Jenn