The Mid-Continent Public Library has some great readers advisory stuff going on. Two of my favorite features from their site are Series and Sequels and Based on a Book.
Series and Sequels allows the user to find out what's next in a YA or Children's series. Since some series never really number their books and trilogy's such as the Maximum Ride series keep getting made longer, this is a quick and easy way to find out what's next or make sure you actually own an entire series. Users can search by book title, author, series title or subject.
Based on a Book lets readers find out if the book has been made into a movie, thus allowing the neverending, always heated and amusing for spectators "is the book better then the movie" debate to continue.