Here's a page where participants can request reviews of specific books or recommendations by genre and/or age group.  So, for example, one might request recommendations for a fourth-grade unit on historical fiction, or for a ten-year-old who is awaiting the Harry Potter and the Deathly Hallows.

• I started this wiki after a conversation about the difficulty of finding good historical fiction for fourth-graders. Suggestions are welcome, especially for books that would work well in book groups. As I understand it, kids read easier material in their book groups than they would read independently.  (Lisa Gordis)
• I'm also always on the lookout for books that will be of interest to my Harry-Potter-loving sixth grader. She's a strong reader, but isn't particularly interested in teen books that focus on romance. Fantasy and magic are her favorites, though she also likes historical fiction, realistic fiction with some humor in it, and mysteries. Her most recent favorites have been Jurassic Park, Treasure Island, and The Outcasts of 19 Schuyler Place (Lisa Gordis)

This page never really took off, but you're welcome to add to it.