For Caribbean octocorals I've found Reef Coral Identification 2nd edition by Humann/Loach ISBN 1-878348-32-9 informative. It's written from the perspective of underwater identification, not the reef hobby. The information can be useful to a reef hobbyist, however: where the specie occurs, at what depth, how common it is and what the average colony size is. Of course it's just Caribbean gorgonians that are collected/imported, unless you get an incidental coral on cultivated rock. Coralimorphs, anemones, and sponges are in a separate book, Reef Creature Identification 2nd edition ISBN 1-878348-31-0. I have the three-book set that includes Reef Fish Identification. A good resource IMO if you are interested in stuff from the Caribbean.
People will differ on what is "good": a well-known book that has been around over a decade is Aquarium Corals, Borneman. This is a very basic genus-level ID book that will cover most of the genus of octocorals you are likely to encounter in the trade with advice on husbandry. It was a very popular book when it was first released, don't know why it has never been expanded/updated as it has become outdated.