Ron Zacharski is currently writing a data mining book, A Programmer’s Guide to Data Mining. The book is targeted at programmers that want to know when and how to apply recommendation engines and other data mining techniques. The book is still in the writing phase, but I can say the first couple chapters are excellent. The book will always be available for free download.
If you are a programmer that is looking to add some recommendations to a website, I would highly suggest taking a look at this book.