Our DS6 reads with good comprehension at at least 3rd-4th grade level, but, like others, often prefers books with pictures to help draw him through the narrative. He's been burning through my old childhood collection of Peanuts books - which are not challenging linguistically but actually have some fairly mature relational content.
We are going to order some of the French Tin Tin books (in English) to see how he handles them. I see the Geronimo Stilton books at Amazon, but can't get a look at the contents. What general reading level are the Stilton stories at?
Our honored guest (DS) also runs up and down the maturity scale at dizzying speed, by the way. Such an interesting fellow.