Besides your catalog, Amazon, and other book-rich retail sites like Alibris and Barnes and Noble, there are a number of sites specifically designed to help you decide what to read.
Lexile's Find a Book
Reading Rants! Out of the Ordinary Teen Booklists
Stop, You're Killing Me (Mysteries)
What Should I Read Next? Simply plug in a book you like, and this site will kick back a list of recommendations.
What's Next? This is the perfect one-stop, no-frills source for series information.
Whichbook.net uses mood and appeal factors, not plot descriptions, titles, authors, or subject schemes.