Sir Circumference and the Dragon of Pi by Cindy Neuschwander, illustrated by Wayne Geehan. Sir Circumference is a silly story that uses finding pi as a solution to a problem: the Knight Sir Circumference has been turned into a dragon. Characters named Radius and Lady Di of Ameter help him by finding a potion with poetic instructions telling how to find the right dosage (circumference/diameter) to return to human form again.
Radius explained what he learned and what pi means and names this number pi. The illustrations and the story are engaging, and although the focus on circles is slight, it does teach what diameter and circumference mean and introduces pi for the teacher interested in introducing pi only briefly.