Ooh, I've got to get my son into L'Engle. Right now he continues to read lots of comics. The Geronimo Stilton books look like they might be right up his alley interest-wise, and I have had "George's Secret Key" on my wish list for a while too. Lots of great suggestions in this thread.
The highest-level book he's reading right now is "Ender's Game", where he's a few chapters in (it's our bedtime book, which he reads to me). I have to find time to read ahead in that for him, since IIRC there are some challenging topics (e.g. animal torture) that I may not want him to read about. I'm surprised by how easy the vocab in that book is (making it a great one for learning spelling), although I think the content, structure and near stream-of-consciousness bits make it a plenty challenging read for him technically and otherwise. He's also enjoying the Narnia books right now.
