I adore How to Clean a Hippopotamus-- not long, but beautifully illustrated and rich in facts.
Have you ever looked at Visual History of the World type books? (
http://www.amazon.com/National-Geographic-Visual-History-World/dp/0792236955/ref=pd_sim_b_2 ) National Geographic Books has many of this kind of thing, and they have excellent sales every so often. Too dense for a sustained read for your DS's age, but maybe good to dip into. They have science books too.
I bought a set of 6th-grade science books from Prentice-Hall (got them on Alibris for $1 each) a while back and both my kids have enjoyed them.
Our elder DS at that age was mesmerized by the Guinness Book of World Records-- Scholastic carries bowdlerized versions (still worthy of a pre-look by you).
Will keep thinking.
DeeDee
