My library currently consists of Sprung and Delbeeks "The Reef Aquarium" vols 1 & 2, Tullock's "Natural Reef Aquariums", Sprung's 'Corals - A Quick Reference Guide", Borneman's "Aquarium Corals - Selection, Husbandry, and Natural History", and Kuiter's "Seahorses, Pipefishes and Their Relatives". All geat books, but I feel like I've "outgrown" Tullocks book, I use "The Reef Aquarium" series for reference, and refer to Bornemans the most often for up to date general info.
Whatever books you buy, make sure that it is either a good recommended reference book, or a book that has recently been published. There have been a lot of recent advances in this hobby in the last few years, and you need books that address them (CA reactors, DSB, MH and HQI lighting, quality products vs. crap, refugiums) It's great stuff to read while you're waiting for a new tank to cycle or for LR to cure.
Mitch