Sir Cumference + The Dragon of Pi: A Math Adventure by Cindy Neuschwander and illustrated by Wayne Geehan is a very useful and entertaining story about a knight who accidentally drinks a potion that transforms him into a fire-breathing dragon.  Sir Cumference needs to find a cure, so his son Radius goes to find a spell or potion to transform his father back into a human. Radius finds out that he needs to find the ratio of the distance across and the length around him, and he tries to find out how to measure it. He figures out that the ratio is 3.14, and he needs to give his father that many spoonfuls of medicine to turn him back into his human form. I think that the theme in this book is perseverance. I think this because Radius only had less than a day to find a cure for his father, but he tried very hard and eventually found out how to cure his father. This is a very good book because it has the perfect mix of excitement and teaching moments, such as using pi, diameter, radius, and circumference. It almost sneaks the math terms into the story, so that we do not think of this book as a math book, but a normal story with math elements. I think that this is a good book for 11-12-year-olds because they are starting to learn about circumference and diameter.