This book is thorough in its approach to educating you on the why's and how's of using PhotoShop for image processing. If you've always wondered how to best use PS to process your astronomical images, this book will give you the nitty-gritty details of how to do it, as well as why you should do it.
The text is split into logical chapters, so that if there are areas that you aren't interested in, you can easily skip them.
This is a GOOD book! It's not inexpensive -- but it's very comprehensive.