There is a time warp trio book on Mayans which focuses on their numbering system, including zero and its advantages - could be a cool one if he hasnt already blasted through that series.
I remember when I read this one to ds in 1st grade, the teacher reported having to ask him not to write his math work in mayan...again, I am the crazy mom who thinks we should be encouraging these sorts of explorations - LOL!

You might find a chapter or two in books on Eyptian or Mayan culture and history which might suit.

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