Okay, wack me with a wet noodle but I do recommend Take Me Back: A Trip Through History and One Million Things: A Visual Encyclopedia. These two books are very thick volumes so may be somewhat different from the little thin kiddie books you usually see in the kids' section. I am not sure what the reading levels are but maybe 5th grade level but there are lots of graphics.
I also recall some books from the Brainwaves series that were interesting. The Guiness Books of World Records are always a favorite. There are also versions by Scholastic that DS really liked from an early age.
