Oh my. I think I know what I'm doing this weekend. LibraryThing is an online personal library catalog with social networking tossed in. I added a few books to my shelf, just for fun, but probably should do something billable today, so I'll leave the rest of the exploration for tomorrow. It looks like once you get your booklist put together, you get linked to other people's libraries based on common titles. And there are about a zillion discussion groups set up. It would appear to be one big happy recommendation list.
If it turns out to be from the Devil, I'll let you know.