The BEST book (or reading material) for any photographer, underwater or not, is THE CAMERA MANUAL. Read it, practice it, understand it, practice it, play with it, and did I mention practice with the camera?
In addition, if there is a book specifically for your camera (i.e. "X" for Dummies), get it.
There are really two different, VERY different, aspects of using any camera. The first, and probably the easiest to "master" is the camera itself -- how does THAT PARTICULAR camera work. Really, all that takes is reading the manual and playing with the camera.
The most important aspect is "the eye behind the lens" -- that is, what is it that YOU want the image to be. That takes knowledge of the camera AND that special "something else" that some people have in spades (and I, unfortunately, do not, sigh). But even if you are like me (i.e., one who is artistically challenged) you can help yourself by looking at pictures and studying them. What do YOU like about the picture -- what don't you like about the picture.
A fun source of information, and free, is Underwater Photography Magazine --
UwP = Your free Underwater Photography Magazine | Home page
There are also numerous websites such as --
Underwater Photography Guide -- that have images, tutorials, etc.
Good luck and have fun.
BTW -- underwater photography tends to take a lot of a diver's bandwidth so work on your diving skills a lot before getting too engrossed in taking pictures. Your diving should really be automatic before sticking your eyeball behind a lens.