For starts and then adding on down the list (important ones);
Reef Invertebrates, Anthony Calfo
Aquarium Corals, Eric Borneman
Invertebrates, Julian Sprung
Coral Reef Animals of the Indo-Pacific
Baensch Marine Atlas, Vol 1,2 & 3
Indo-Pacific Coral Reef Filed Guide, Gerald Allen
Tropical Pacific Invetebrates, Patrick Colin
Reef Fish, all volumes, Scott Michael
Corals of the World, Vol, 1, 2 & 3, Jen Veron
Filed Guide to Anemonefishes and their Host Sea Anemone, Daphne Fautin
Soft Corals and Sea Fans, Katharina Fabricius
Corals, Julian Sprung
Then things start to go to more specifc groups, I'll just throw out a few of mine or I would be here all day
Micronesian Reef Fish, Robert Myers
Fishes of the Great Barrier Reef and Coral Sea, John Randall
Corals of the World, Elizabeth Wood
Invertebrates: Tube-, soft and Branching Corals, Peter Wilkens
Invertebrates: Stone and False Corals, Coalonial Anemones, Peter Wilkens
Nudibranchs and Sea Snails, Hlemut Debelius
1001 Nudibranches, Neville Coleman
Crustaceans of the World, Helmut Debelius
Sea Stars, Sea Urchins and Allies , Gordon Hendler
Coral Reef Ascidians of New Caledonia, Claude Monniot
Marine Faltworms, Leslie Newman
Caribbean Reef Plants, Diane Littler
South Pacific Reef Plants, Diane Littler
Filed Guide & Atlas of the Seaweed Resources of the Philipines, Gavino Trono
If your are lucky enogh to find a copy without costing an arm and a leg
Marine Life (inverts only) , David Geroge ( our first real ID book on invertebrates)
Readers' Digest Book of the Great Barrier Reef (our first book brought into this hobby that explained things, it is our Father book of the hobby). The full sized one 385 pages
I have a couple hundred ID books