I babysit a kid about this age, and I'm always surprised by what he's reading with his parents.
One of his favorite series is the
http://en.wikipedia.org/wiki/Warriors_(novel_series) He reads these on his own. With his parents, however, he's tackled philosophy, [I]Lord of the Rings[/], literally whatever he wants to read that's above his reading level. They read
Lord of the Rings when he was 6 because E. wasn't satisfied when they finished off
The Hobbit, and he heard from me that there was more Tolkien to be had.
Right now they're working their way through all of the Harry Potter books, mostly because Mom wants to reread them too.