Essential Underwater Photography Manual is my favorite overview book about underwater photography. Written by Larry and Denise Tackett and released in 2007 it covers a lot of the most current equipment and best practices.

It starts with a good introduction to photography which many people in the digital point and shoot realm never get.   It also goes into many of the unique issues you’ll face when shooting underwater. Such as:

• Distance to your subject
• Lighting for macro and wide angle shots
• Depth of Field and Point of Focus

There are a lot of great tips and tricks for taking various types of pictures both macro and wide-angle.