The Librarian Who Measured the Earth by Kathryn Lasky
Very excellent book. It took us several days to read it as the text is deep. The math was over their first grade heads, but they still really enjoyed the story. The illustrations enhanced the telling as well.
This tells the story of Eratosthenes, the ancient Greek librarian who figured out how to calculate the circumference of the earth.
We will read this again for both middle and elementary study. I wish more books were like this.