A few years ago, I bought the Audubon Society Field Guide to Tropical Marine Fishes and found when I tried to use it in the Florida Keys that there were many fish I couldn't identify. People recommended I buy Paul Humann's book, "Reef Fish Identification." The book is more expensive, but I found it to be far more comprehensive and user friendly. For example, many fish look entirely different when in their "juvenile," "initial" or "terminal" phase, and the "Reef Fish Identification" book has clear photos of each of the three stages shown beside each other, and frequently includes photos of alternate color phases as well. The Audubon book usually just shows a picture of the fish in just one phase, and often not a terribly good photo at that. Next to each photo in the Humann book is an excellent line drawing of the fish, highlighting and labeling which features of a fish are most dependable for identification. The Audubon book includes no such diagrams to aid identification. Finally, the Humann book is based upon both an extensive bibliography and collaboration with field biologists, and if the detailed descriptions in his book of definitive features for discriminating species of fish are correct (and experienced scuba divers tell me they are correct) then some of the pictures in the Audubon guide are actually even misidentified as to species. For example, the Audubon picture labeled as a "Leopard Goby" is almost certainly a picture of an "Orange-sided Goby" (if the Humann book is correct about dark lines outlining the orange rectangles being a reliable discriminating feature). If there were no other reef fish identification book available, I would have given the Audubon book more stars, I am usually a great fan of the Audubon Field Guide series, but in comparison with the Humann book it just doesn't rate very well. I should add that I have never met, nor have any private or professional association with, the author of either of these books, so my opinion is not biased by any such affiliation.
Finally, I should add that I am pleased nevertheless to own both of these books because, for many species of fish, fish of the same species can be found in a wide variety of colors and patterns, so you can never study too many photos to get a handle on the range of appearances possible for any given species. Buy the Humann book if you can just afford one book, but owning both books is even better!