The invisible web, or deep web, is estimated to be 500 times bigger than the searchable web and comprises databases and results of specialty search engines that the popular search engines can't index.
You sign up, enter the name of a book, then find out what other folks who liked that book also liked; a tag cloud for the book leads to other books on this topic. Quite addictive, and I just used it to look at the info. around one book.