Shooting with any kind of flash completely changes the photographic experience. No longer do you simply need to be aware of light; you need to know how to shape it to your own ends. This guide covers the details of setting up and using modern TTL flash systems.
I really enjoyed this book. It lays out very clearly all the fundamentals of flash photography with various light modifiers.
However one thing alluded to in the book but really strongly pointed out to me by Gregory Heisler on workshop recently was that exposure for the environment or background is controlled by shutter speed and exposure of the subject in flash is controlled by aperture.
Heisler did a very good graphic demonstration of this by setting the aperture correctly for a subject in flash then varying the shutter speed in subsequent shots. The effect was that the subject stayed correctly exposed in exactly the same way in all shots and but the background lightened and darkened in accordance with the increased or decreased shutter duration. This point is also made in this book but not so strongly stated and demonstrated.
Regardless of this I would highly recommend this book as a very good introduction to flash fundamentals.