One of the most widely recommended books on that topic is, "Photoshop: Restoration and Retouching," by Katrin Eismann, ISBN 0789723182
This is a fantastic (!) book. She gives great examples, teaches well, and you can get the images used in the book off the website to follow her methods.
Jeff Spirer
Photos: http://www.spirer.com
One People: http://www.onepeople.com/
Surfaces and Marks: http://www.withoutgrass.com
