If you’re a serious diver, you want to know what you
see in the deep. The only way is to take a good book with
you to identify the fish and study their behavior. But don’t
rely on your dive resort or liveaboard to have it on hand—
most stock a few dog-eared copies, but they may not be
current or complete, or else another guest may have taken
the book you need to his room.
Here are the best Caribbean and Pacific Coast ID
  books to add to your reference library. I’m never without
  these books when I travel to those sites, and I refer to them
  when I write. You can find longer descriptions on our website
  at www.undercurrent.org.  
Buy these from Undercurrent by going to the book page
  on www.undercurrent.org. You’ll get them for the price
  currently listed on Amazon.com, although they are subject
  to change. All the profits from book sales go directly to
  programs that are saving coral reefs.  
The Caribbean  
The Reef Set: Reef Fish, Reef Creatures and Reef Coral,  by Paul Humann and Ned Deloach. This three-volume set
  is the unparalleled source for information on Caribbean
  sea life and identification. Because so many species of
  fish continually confuse divers with their variable colors
  and markings, the Reef Fish Identification’s third edition
  (512 pages) covers them all. The Reef Creatures book (488
  pages) covers sponges, nudibranchs, octopus, crustaceans,
  Christmas tree worms and more. The Reef Coral book
  (252 pages) helps you identify all the hard and soft corals,
  spawning, and even the growth on top of corals, as well as
  algae and other plant life. Each book retails for $40, but
  the boxed set is discounted by 37 percent. Hardcover with
  shelf case, 7 x 10 inches, $75. Paperback with weatherresistant
  canvas bag, 7 x 10 inches, $85.  
A Guide to the Coral Reefs of the Caribbean, by Mark
  D. Spalding. This book doubles as a guide to the natural
  history of coral reefs and a diver’s travel guide. It covers 35 dive destinations with key information on reefs, marine
  parks and remote places, and a photographic field guide of
  the marine flora and fauna. Spalding, a coral reef scientist,
  delves into eco-problems with a focus on what each person
  can do to protect the reefs. Paperback, 7 x 10 inches,
  $24.95.  
Pacific Coast of Mexico and Central America  
Fishes of the Tropical Eastern Pacific, by Gerald R.
  Allen and D. Ross Robertson. The ultimate ID book
  for the Baja, Costa Rica, the Galapagos and the Sea of
  Cortez. Photo-packed pages cover 680 species of sharks
  and sailfish, wrasses and razorfish, pipefish and pearlfish.
  Sponsored by the Smithsonian Institute, Drs. Gerald
  Allen and Ross Robertson’s definitive volume describes
  and comments on these critters’ remarkable behaviors.
  Hardbound, 8.8 x 11.5 inches, 332 pages, $85.  
Sea of Cortez Marine Animals, by Daniel W. Gotshall.
  The book you’ll need in order to identify critters anywhere
  along Mexico’s Pacific Coast, all the way to Panama.
  Gotshall, a marine biologist with 34 years’ research experience,
  has more than 250 photos of fish, corals, nudibranchs,
  lobsters, sea stars, and other critters endemic to these
  waters. There are tips on how to identify each animal and
  where to spot it. Paperback, 7 x 9 inches, 112 pages, $21.95.  
Reef Fish Identification: Baja to Panama, by Paul
  Humann and Ned Deloach. The latest edition in
  Humann’s series of marine life books, and the most comprehensive
  field guide for identifying reef fishes from
  the Gulf of California to the Pacific coast of Panama,
  including offshore islands. More than 500 photographs
  of 400 species in their natural habitat. Each species’ portrait
  includes the fish’s common, scientific and family
  names, size range, description, visually distinctive features,
  preferred habitat, typical behavior, depth range, and
  geographical distribution. Paperback, 6.5 x 9 inches, 364
  pages, $39.95.