I just came back from a dive trip to Bali and spent a week, off and on, IDing the fish I'd photographed. This book turned out to be the most useful. It includes photos of a lot of color/sex/age variations that proved invaluable. This book also puts little arrows in many pics to point out features that help differentiate a fish from otherwise similar ones--I found that to be very useful.
However, though it's the best book for tropical Pacific fish IDing, I did find that I needed to cross-reference it with several others in ambiguous cases (and many cases were ambiguous!)--particularly one Aussie book that uses paintings instead of photos: "Marine Fishes of South-East Asia" by Gerry Allen (though this book often uses different common names, so you'll have to go by scientific names in many cases to correlate it with American texts). Sometimes a painting can highlight features a particular photo won't show clearly. And this book shows some interesting fish that aren't strictly coral reef fish, which nevertheless you might see on a trip (think mahi-mahi, flying fish etc.).
Lastly, you should also have a general underwater guide, for 3 reasons: (1) this is what you should actually bring with you on a trip, leaving larger, heavier, more specialized books like the one being reviewed here at home (especially with current luggage weight restrictions). (2) A general guide, such as "Indo-Pacific Coral Reef Guide" by Dr. Gerald R. Allen & Roger Steene, also has everything from corals to sea snakes. (3) Any given fish you see is probably a common one; a general guide will only show common fish. So you'll generally want to look here first.
The book I'm reviewing here is organized for identification rather than scientifically. It uses 20 ID groups, such as "disk-shaped/colorful" and "odd-shaped bottom dwellers." This is appropriate since it's a fish ID book.
If you dive in Pacific/Asian waters--and that's the best diving on Earth in my experience--and you'd really like to know what the heck you saw--get this book.