While the fish are often spectacular on the reef, it is the invertebrates that are often the most fascinating creatures to be found. Life in the sea allows for a bewildering array of body shapes and lifestyles. The creatures found by divers will often be things that stretch the imagination. The unexpected is common and the observant diver will find life forms both beautiful and horrifying.
The guide to invertebrates on the reef is Hawai‘i’s Sea Creatures, by John Hoover. Written for the non-professional there are over 500 species presented, all with excellent photographs. Each species is resented with a short paragraph or two summarizing what is known about the creature. To supplement the books, the author maintains a website with updates that can be checked when the book fails.
For someone like myself who spends much of a dive looking for the small creatures that most divers overlook this book is incredibly useful. I received the latest edition under the Christmas tree and it is often the first book I reach for after returning from a dive.
I have come across a few species not in the book. But the book will at least get you close, there will be something similar allowing you to identify the family or maybe the genus that your unknown critter belongs in. For anyone diving in Hawai‘i, this is one book that is indispensable.