This is a good NONTECHNICAL intro to flash photography. It's approachable and an easy read, with some good examples and image descriptions. You'll learn more from Joe McNally, Zack Arias, David Hobby and others, but this isn't a bad starting point. I borrowed this copy from my library and as such, it was a fine use of time.

As a side note: Many of the model images the author used were a bit on the cheesy side for my tastes.