What specifically are you trying to do and what language are you using? <p>For Python, I like NLTK at <a href="http://nltk.sourceforge.net" rel="nofollow">http://nltk.sourceforge.net</a><p>Java has something similar, though I've not used it, at <a href="http://opennlp.sourceforge.net/" rel="nofollow">http://opennlp.sourceforge.net/</a><p>There are tons of other Java libs around, per usual. <p>I know of a few Lisp and C/C++ libs, but I've never used Ruby or PHP, etc. <p>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.