Idiot's Guide to Sign Language

I've been meaning to learn at least a little sign language for a long time. This book has been an excellent gateway to that skill. In the accessible manner that's made the "Idiot's" series famous, the book goes through the alphabet, numerical system and basic grammar of American Sign Language.
It also highlights some of the unique qualities of ASL; it's not just signed English, for example, nor is it finger-spelling. It's a robust language all its own, complete with history, regionalisms, quirks and puns. It will by no means make one fluent, but it's an excellent introduction to ASL.