I'm an advanced hobbyist photographer. I have Canon bodies and flashes. I've found lots of general flash resources that have all been somewhat helpful. However, the missing link is a book dedicated to Canon flash devices.
The author does an excellent job of examining the Canon system and of explaining the features of the flashes.
That said, this isn't a replacement for the manual; it doesn't go over every button press and configuration option. Instead, there are a lot of examples showing how to use the flash and its features to get better pictures. There's lots of before and after photos to show how a feature changes the resulting image.
If you've just bought your first Canon flash, you might feel that this book assumes you have too much knowledge that you don't already have. However, if you've already learned the basics, this book will teach you some techniques that you won't easily find elsewhere, and as it does it will also show you how to set up your flash (and sometimes, Canon camera) to enable them.
I've uploaded a picture to this page that was taken using a technique learned via this book. The link is near the picture of the cover of the book at the top of the page.