If you're looking for reading material, check out the recommended list for prospective instructors.
A great resource on casting, however, much of it is disorganized or out of date. If you have a question on casting, try searching the board history.
The best place and time in the world to learn about casting. See you in West Yellowstone August 24-28, 2010.A good resource to study for the FFF instructor examinations.
Great fishing photography magazine from Brian O'Keefe.
Jason Borger's Blog, interesting in it's own right, but includes some excellent motion capture footage of fly casting.
The FFF instructor's magazine.
A collection of casting games.