It sounds like you are talking about a naive Bayesian classifier. PG wrote a couple of articles on his experience with these for spam filtering (http://www.paulgraham.com/spam.html and http://www.paulgraham.com/better.html). They're probably a decent high-level introduction to the area.

For a more in-depth, yet very accessible discussion, I would recommend "Speech and Language Processing" by Jurafsky & Martin (http://books.google.com/books/about/SPEECH_AND_LANGUAGE_PROC...). It's considered by many to be the Bible of NLP.