Aquarium Corals by Eric Borneman.
Conscientious Marine Aquarist
The Complete Encyclopedia of the Saltwater Aquarium.
Those are probably the three I read the most. I have a stack past my knees, but those three I find to be the most helpful expecially when I cross reference. I have still not found an encyclopedia of nothing but corals which has all the answers, but the first book listed is pretty darn close. Hope this helps.