I was really impressed with the Photoshop Astronomy book. It is a fantastic resource. And not just if you are just starting out.
I also have the Jerry Lodriguss book (well disk actually) "Photoshop for Astrophotographers" and it does contain great directions, as do many other online resources, however I am one of these older farts that likes to have a paper manual beside me when I'm working on something.
Also re Jerry Lodriguss' PfA, a considerable part of the disk discusses colour, colour spaces (which is good) and film photography which is great if you want to start from scratch and get a good understanding of the imaging etc. But it is not primarily a digital imaging guide. The Photoshop examples are very much cookbook style techniques which is great, but only one chapter in 5 is to do with photoshop. Basically the chapters are
1. Colour
2. Astrophotography (setting up, equipment, techniques etc)
3. Scanning
4. Photoshop
5. Archiving
If all you want is the
techniques then you will get them off his website anyway.
My preference is definately the Photoshop Astronomy book.
HTH