Would it be wise to take an underwater photography course (such as the one PADI offers)?
In addition, is there a good, general, UW Digital photography book members like out there?
In my experience, the underwater photography course offered by PADI was virtually a complete waste of my time and money. I admit that I took it years ago in the pre-digital age when I was shooting with my Nikonos V film camera, but the instructor didn't teach me much of anything about uw photography. It seemed to me it was just a series of exercises for me to do. In the end, I felt like he was just taking my money, sad to say.
I did get a lot out of a workshop I attended last year in Cozumel with other uw photographers and a qualified instructor.
As far as books go, I highly recommend the Martin Edge book that several have mentioned. I got it out of the library, then liked it so much that I requested it as a Christmas present so I could refer to it again and again. Oh, and I received it as a Christmas present 
Another book I picked up while shopping for something else at the local bookstore: "The Essential Underwater Photography Manual" by Denise and Larry Tackett. I did not like this book nearly as much as Martin Edge's book and found it to be quite a bit more basic, but still found it somewhat helpful.
Ellen