For learning and mastering an impressive package like Photoshop 6 I usually say that you need 3 kinds of materials: 1. A beginners guide to show you the framework and teach you how the basics work; 2. A detailed reference volume for when you need to research the solution to a problem on your own; 3. A selection of impressive advanced effects to give you inspiration and solutions to common advanced problems. If you have access to these 3, all you need is sweat and time. Few books have it all, since many of these functions seems to contradict with each other. Another common flaw is that many books show you very impressive effects, and how to carry them out, but after doing so, when the initial "Wow! Look what I just made!" fades away, you have learned almost nothing and understand very little about how the effect was made.
Based on these criteria, I have to conclude that The Photoshop 6 Wow! Book has it all. This is definitely not my first book on Photoshop 6, but I wish it was. It would have saved me quite a few bucks, and taught me more in the same time.
My personal interest is creating illustrations, not so much applying effects to photos or making web graphics. But still the Wow! Book covers it all to a very satisfying degree, in fact with so much depth and detail, that this book doubles as my reference volume. Also it patiently takes its time to make sure beginners understand all the key concepts in Photoshop.
I am impressed. Very impressed. My verdict will have to be the old cliché about, "this is the only book you'll need". But it can easily be the only book you'll need for a loooooong time.
Feel free to mail me if you have questions.