My kids recently re-discovered this website that we've enjoyed before: BookAdventure.com It suggests books by reading levels or genre. Kids can take quizzes on books they've read and they earn points. Though we've never redeemed points, the girls seem to enjoy taking the quizzes.
Here's their intro:
Book Adventure is a FREE reading motivation program for children in grades K-8. Children create their own book lists from over 6,000 recommended titles, take multiple choice quizzes on the books they've read offline, and earn points and prizes for their literary successes. Book Adventure was created by the Sylvan Learning Foundation and is sponsored by Sylvan Learning, Inc.