I can't quite remember where I originally found this link but check out all the research papers on collaborative filtering. Many are freely downloaded though some are behind the ACM curtain. I think I really need to splurge and get a membership because there is a ton of great information available there.
A very interesting one is by Pattie Maes, Social Information Filtering: Algorithms for Automating "Word of Mouth". If you don't remember Pattie Maes, she started Firefly and then it was sold to Microsoft which I don't think every really used it.
Technorati Tags: socialnetworks, tags, recommendation+systems