"Adobe Photoshop Restoration and Retouching" by Katrin Eismann is excellent, if you're already familiar with Photoshop. It's the book I see most often recommended on RetouchPro, which is a dedicated retouching forum. "Skin: The Complete Guide to Digitally Lighting, Photographing, and Retouching Faces and Bodies" by Lee Varis is also pretty good, and has a very useful section on colour correcting by numbers.
It's also worth checking out RetouchPro -
http://www.retouchpro.com/ . The forums are great and some of the tutorials have techniques which I've never seen anywhere else.*
Cheers,
Tim.