New World Publications makes some of the best identification books for identifying marine life. Their 3 volume set on Carribean Reef Fish, Reef Corals and Reef Creatures has long been the gold standard of learning about all the living things underwater in this part of the world.
More recently New World Publications has ventured into more specific subjects such as Nudibranch, the huge compendium Colours of the Reef, and guides to Pacific Reef Creatures and Reef Fish. The new 2015 2nd edition of Reef Fish Identification: Tropical Pacific is particularly great as it features lots of new pictures of rare and unknown species with more recent observations about natural distribution and habitat preferences.
Better yet, the whole line of marine life identification books from New World Publications is available both in ebook and PDF downloads. Most of the physical ID books from NWP cost in the range of $45 with the digital version being ten bucks less at $35.
Not only are the digital versions of these books much easier to carry as a traveling fish geek or underwater diver, but they are also searchable making it easier and faster to data mine about the fish or critter you are interested in, or just quickly getting to the kind of fish you are seeking to look up. The digital versions of the various books by New World Publications come in formats that are fully compatible with a wide range of mobile and desktop devices, and are available to purchase online. [New World Publications]