Advice on development and useful books

Hi all,
The company I work for develops a fraud detection application for the telecoms industry. We have a pretty efficient rules and thresholds based system working and I have been looking into the next stage of development for it. My research indicates that the next stage should be to develop a signatures module. The papers I have been looking at have some pretty heavy maths going on in them that I would really like to understand. Some of it is statistics based, I think there is some set theory as well. I do remember how to calculate standard deviation for a population but apart from that it is a long time since I have done this type of maths. Can anyone please point me to a good books that will get me up to speed?