I don't use much for books but, Ari really likes the signing time series:
www.signingtime.com
I find it easier to learn the signs from a DVD then from a book. The pictures can sometimes be difficult to interpret. We usually pick a sign and use it a lot trying to help her make the connection between the sign and the object/concept.
For example, it's really helpful if she can tell me if she is "All Done" or wants "More". So during meal times when I offer more I sign it and when I take away her plate I sign All Done.
This web site
http://commtechlab.msu.edu/Sites/aslweb/browser.htm has video clips of some of the signs.
You'll also want to see what is available in your local community. See if you have any community ed classes in sign language. You may also want to look for other people that use sign language, both for you and for your son.
