Fishdb Shop - UK/US
We highly recommend the Humann/Deloach books as well as the Debelius books for their good layout and fantastic photography. The Ewald Lieske book "Coral Reef Fishes" has drawings rather than photographs but it covers the largest number of species of any of the references we currently use.
If you know of any other great reference books, please let us know.
There's a dvd video/guide obtainable online called Marine Life of Fiji and Tonga produced by people that have helped us with identifying pictures on this site.
Recommended by another contributor to the site is this book on Coral Reefs.