If you're after second-hand books (one of my obsessions ), both www.bookfinder.com and www.addall.com are well worth a visit. They do have a keyword function but it's not wonderful. Author or Title search work well. The only minor problem with both sites is that the vast bulk of the bookstores are in the US (well minor problem for us Europeans), but I've never had a problem getting a bookseller to post to the UK (generally at a reasonable rate), and I've also ordered books from Sweden, Australia, Scotland and other countries.

Richard