Sir Cumference and the First Round Table by Cindy Neuschwander, illustrated by Wayne Geehan
King Arthur is convening his knights for the first time, but there is a problem -- the table. Sir Cumference takes it upon himself to solve the problem.
This children's book helps children in grades 2-5 see the connections between various polygons and a circle. The properties of each polygon are used in the story. Mathematically it is sound and the author does explain the material in a very nice child-friendly manner. Adults will enjoy the plethora of puns.
My only complaint is the colors in the illustrations is a bit too wild -- tone down some of the contrasts would help the appearance.