Sir Cumference and the First Round Table A Math Adventure by Cindy Neuschwander illustrated by Wayne Geehan*

We read this book in honor of National Pi Day.
My twelve year old groaned at the word play, but still enjoyed it!
I think the definitions and concepts were cemented in my nine year old's mind after reading this fun story.
King Arthur and his knights gather to plan how defeat an impending enemy attack.  The shape of the table, however, impedes their ability to discuss and plan.  Sir Cumference and his wife Lady Di and his son Radius offer a solution that pleases everyone.
*Highly recommend.
-Borrowed from the library.