If your library subscribes to it, try NoveList. It's a database that provides similar books based on title, author or genre.
Fantastic Fiction is a site that a fellow librarian introduced to me and it's great as well. You can search by author, title or ISBN. If you search by author, you'll get a list of books written by the author as well as some read-a-likes.
http://www.fantasticfiction.co.uk/