This is a medieval story about Radius, a boy who wants to become a knight. He trains under Sir D’Grees and his father is Sir Cumference. Radius is sent on a quest to find King Lell, of the neighboring kingdom. As a gift he receives a family heirloom (that is actually a protractor). Radius travels through a cute village and the Mountains of Obtuse to reach a castle with a riddle on the door. The riddle involves angles. As Radius makes his way through the labyrinth inside, he must measure angles to find out which way to go. He eventually finds King Lell and discovers that the dragons chasing him in the labyrinth are King Lell’s pets, a “pair of Lells.” Radius comes back, names the angles he has discovered, and is knighted for bravery and intelligence. King Lell renames the kingdom “Angleland” (England).
Besides this book, there are five others in the Sir Cumference series: Sir Cumference and the First Round Table, Sir Cumference and the Dragon of Pi, Sir Cumference and the Sword in the Cone, Sir Cumference and the Isle of Immeter, and Sir Cumference and All the Kings Tens. Though the illustrations are lovely, the books are meant for older kids (upper elementary) who will understand the humorous wordplay with math-sounding words and phrases. The Great Knight of Angleland in particular is well written and clever, a fun way to introduce or refresh the topic of angles in a math class. Math storybooks are not common, and this one is very well done.