What specifically are you trying to do and what language are you using?
For Python, I like NLTK at http://nltk.sourceforge.net
Java has something similar, though I've not used it, at http://opennlp.sourceforge.net/
There are tons of other Java libs around, per usual.
I know of a few Lisp and C/C++ libs, but I've never used Ruby or PHP, etc.
As far as reference books, I mainly just use online documentation and references (Google it.) I also like an older Lisp book by Mellish, but it doesn't use the latest techniques or anything.