[Delphinus]

Kind of pricey but "The Reef Aquarium" by Julian Sprung and Charles Delbeek, volumes I, II and now III are really the bible of reefing.

Fossa and Nilsen also have a similar series, from a European perspective, said to be very good as well, but I haven't read those myself. There is said to be a lot of overlap between this and the TRA volumes.

Clownfishes, by Joyce Wilkerson

Anemonefish And Their Host Sea Anemones, Daphne Fautin and Gerald Allen
(This is the bible of host sea anemone identification, plus anemonefish identification and geographic ranges. Not a whole lot of relevant info for captive care, however, because the authors do not (or did not at the time of writing), approve for the practise of captive anemone keeping. Nevertheless these two are the worlds foremost authorities on the subject so it is worthwhile having, if your into anemones or anemonefish. For captive care, including breeding of clownfish, the Wilkerson book is a better bet. Ultimately, you'll have to have both if this topic interests you.)

Clams by Daniel Knop

Clams by James Fatheree

Knop is/was the authority on tridacnids for the longest time. His book however is slightly dated (yet the info is still relevant), plus it is translated into English from German. The Fatheree book just came out a few months ago so is very flashy and new. Both books are worth having IMO, if clams interest you.

Aquarium Corals by Eric Borneman.

Fish books - pretty much anything by Rudie Kuiter is going to be an amazing reference. Also look for Scott Michael. There are too many titles to list but look for these authors.

There are, of course, so many others. Too many to list at once! These above ones are my favourites but I bet I'm overlooking a few too. My best advice is look to the serious reef oriented vendors such as J&L, Ocean Aquatics, and see what books they sell.

For a starter reference though, look to the TRA volumes (first one I mentioned). Can't go wrong with those.