I don't know the best book, but the ones SWF has are pretty good for the basic hobbyist, so I hear.
Now, if you want to drop a ton of cash, I mean in the hundreds, I have seen some really complex coral books in stores. If you live near a school with a marine biology department, their bookstore usually offers some nice choices.
Ask one of the sharks, they probably have some good suggestions.
All depends on what you're looking for too.
