If you are looking to improve your flash photography, especially vis-a-vis the Nikon CLS (Creative Lighting System), the following two resources are worth a look:
Nikon School presents A Hands-on Guide to Creative Lighting (DVD): This discussion/demonstration of flash techniques, present by Bob Krist and Joe McNally, is over 2 1/2 hours long (!) and is essential viewing for anyone who owns (or wants to own) an SB-600, SB-800, or SB-900. At only $26 on Amazon, this is possibly the best photography investment I have ever made.
The Hot Shoe Diaries: Big Light from Small Flashes (Book): Get the DVD (above) first, but this book is just excellent. Joe's a great writer, very unconventional, conversational, and dynamic, and his discussions and examples are superb. Again, the main focus of the book is the Nikon CLS, though the book would be of interest to anyone exploring creative flash photography.
Happy shooting!