Geronimo Stilton books run at about 3rd to 4th grade level! And they were really huge transitional books for my DS7 in moving from more picture based books to longer, higher level chapter books. He still enjoys the stories and format on occasion.

My DS also read a bunch of Calvin and Hobbes comics and really enjoyed those too. Actually, my DH is supplying him with too many "teen/adult" comics right now!