A bit more obscure, but tremendously fun (and very geeky), are George Gamow's Mr. Tompkins books (http://en.wikipedia.org/wiki/Mr_Tompkins) - I discovered the first two in my high school library, but they would be very accessible to any elementary school child.

Also in the geek mode, why not Abbot's Flatland (http://en.wikipedia.org/wiki/Flatland)?