An intriguing book search engine, sort of: whichbook.net lets you select characteristics of the book you want to read next, then recommends some to you. The main page gives you a set of sliders: happy/sad, funny/serious, safe/disturbing, and so on. You set the sliders to describe your desires, and book suggestions appear. Intriguing. It looked like it did a pretty good job, though I don't know how large a database is has, or how books get added to it.