The "American Sign Language Handshape Dictionary" is the only book I'd consider a "must have" If you're going to buy books that's the first one I recommend buying because I'm sure you're enjoy owning it.
The American Sign Language Handshape ... - Google Book Search
Amazon.com: The American Sign Language Handshape Dictionary: Books: Richard A. Tennant,Marianne Gluszak Brown,Valerie Nelson-Metlay
Be warned; you can't learn ASL from a book, you can build a lot of vocabulary but with a book you don't get much syntax, and most importantly feedback.
Be careful with books from your library, make sure they're ASL and not SEE, they may not specify. Nothing wrong with learning SEE, just make sure you're aware of the differences.