Here's a few sources I go to:
1. Honey for a Teen's Heart, great discussion of reading and gives both maturity and reading level for books, as well as world views of authors
2. Heart of Dakota, Sonlight
3. My book review opportunities (which are mostly done now)
4. Books from when I taught middle school--I have to go back and review these though, because I am finding that I no longer subscribe to the idea that it doesn't matter what kids are reading as long as they're reading. Instead, I want good stuff to go in their heads and junk food isn't always beneficial. A lot of books have the potential to plant dangerous ideas that I feel I have to be careful about when they are introduced to my kids.
I also keep running lists of the chapter books I find for kids of different ages on one of my other blogs here:
If you have any suggestions that I can add to my lists, please let me know!