So while looking for a decent description of The Horse and His Boy, I came across an interesting site called LibraryThing. I haven’t delved too far into it, but it looks like Del.icio.us for books. You can arrange a “shelf” of books (as the call it), tag them with descriptors, see similar books, and find what others who read that book have also read.
It looks like the site launched in August 2005 and has since become the 100th largest library. Might be worth checking out.