I recently posted a thread requesting suggestions for a book on Photoshop CS. I got several nice replies with surprising unanimity in recommending The Photoshop Book for Digital Photographers by Scott Kelby. I bought that book, and I'm quite pleased with it. I'd like to thank all who recommended it. I've only had it a few days and I've already learned much from it. Now I'd like to go beyond Kelby's book. It is a great set of recipes for doing most of the common tasks one wants to perform on a digital photograph, but it is all how and no why. For example, if you want to learn about "levels", it is first mentioned on p. 86 in a prescription for fixing foreground flash effects. It says to press Control-L to bring up the levels dialog and move the output levels slider to the left. Well, that works, but why? What are "levels" and what is accomplished by changing them? So I'm asking for another recommendation, this time for a book which explains the why behind Photoshop tools. I'm sure that the book I'm seeking was included in responses to my previous query, but I'm asking for a bit more specific suggestions now. Thanks again, and I look forward to your suggestions.