Here are the areas I've been researching, some things I've read and some open source packages...

Nearly all text processing starts by transforming text into vectors:
http://en.wikipedia.org/wiki/Vector_space_model

Often it uses transforms such as TFIDF to normalise the data and control for outliers (words that are too frequent or too rare confuse the algorithms):
http://en.wikipedia.org/wiki/Tf%E2%80%93idf

Collocations is a technique to detect when two or more words occur more commonly together than separately (e.g. "wishy-washy" in English) - I use this to group words into n-gram tokens because many NLP techniques consider each word as if it's independent of all the others in a document, ignoring order:
http://matpalm.com/blog/2011/10/22/collocations_1/
http://matpalm.com/blog/2011/11/05/collocations_2/

When you've got a lot of text and you don't know what the patterns in it are, you can run an "unsupervised" clustering using Latent Dirichlet allocation:
http://www.cs.princeton.edu/~blei/papers/Blei2012.pdf
http://www.youtube.com/watch?v=5mkJcxTK1sQ

Or if you know how your data is divided into topics, otherwise known as "labeled data", then you can run "supervised" techniques such as training a classifier to predict the labels of new similar data. I can't find a really good page on this - I picked up a lot in IM with my friend Ben who is writing a book coming out next year: http://blog.someben.com/2012/07/sequential-learning-book/

Here are the tools I've mostly been using:

Vowpal Wabbit (classification and LDA, poor documentation, C++ high performance): https://github.com/JohnLangford/vowpal_wabbit/wiki

Gensim (LDA, vector similarity, text processing, python): http://radimrehurek.com/gensim/index.html

Mallet (classification and LDA, java): http://mallet.cs.umass.edu/

Lingpipe (text analysis, clustering, classification, linguistics, java, commercial open-source): http://alias-i.com/lingpipe/demos/tutorial/read-me.html

Mahout (Hadoop, classification, clustering, LDA, collaborative filtering, java): http://mahout.apache.org/

Langdetect (language detection, java): http://code.google.com/p/language-detection/

Some blogs I like:

http://matpalm.com/blog/

http://blog.echen.me/

http://thedatachef.blogspot.co.uk/

http://www.machinedlearnings.com

MetaOptimize Q+A is the Stack Overflow of ML: http://metaoptimize.com/qa

The Mahout In Action book is quite good and practical: http://manning.com/owen/