This is the definitive bible to CS5. I have read this book cover to cover and it is now on my shelf next to my PC. The best way to understand layers, masks, vectoring etc, is not just to read the book, but to use the cd and practice, practice, practice.
I use this now as a reference book when I'm not sure how to achieve a look that is hard to get.
I cannot recommend this book enough. It is worth far more than the £18.00. So it was money well invested.