A good book for this sort of thing is Toby Segaran's excellent Programming Collective Intelligence. It walks you through this sort of fascinating thing with easy examples and clear explanations. It's sprinkled with simple Python code.

http://oreilly.com/catalog/9780596529321

If you want a good introduction to Naive Bayesian classifiers, there was a pretty readable explanation in Artificial Intelligence: a Modern Approach. It's an expensive book, but I'm sure you can find a copy in any well-stocked university library.

http://aima.cs.berkeley.edu/