PSE8 Books

I tried this in the Retouching forum and got no answers so I'll try here.

I'm taking a PSE8 course that is using handouts. I want to get a book that fills in the blanks. The teacher does a pretty good job of showing how to do stuff but not why I am doing such and such a thing. What would be a good book for that or maybe two books if one won't do it.

For instance today she she showed us how to do curves, but no background on what all of the different buttons do except to tell us that "this is what this button does". I want to know why I'm doing this and more of how do I know to do this to begin with.
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A DSLR is a black hole that you throw money in to, and enjoy every minute of it!!!!