Flash photography was a frustrating mystery to me when I bought this book. Although it provides some basic, much needed information for the complete beginner, it contains a lot of off-subject items (posing your subjects and rules of composition for natural light photographs), but does not cover vital issues such as modelling flash light and gel filters.
This book predates the EX 580 mark II, although the differences between the old and new models aren't so huge.
Basic info is adequate and well-written: GN, wireless flash, basic techniques.
There are some convenient 'crib sheets' for standard flash situations: portraits, macro, event photography, although the amount of detail is necessarily meagre. The images used in the book are, frankly, average.
If you have a basic grounding in flash photography, you'd do better to visit dedicated websites.