The Phantom Tollbooth

Age 7 to 11

The Phantom Tollbooth by Norton Juster and Jules Feiffer

Publisher: Random House Books for Young Readers
ISBN: 9780394820378

In this fantasy novel, Milo finds everything about his life boring... until he comes home one day to find a large toy tollbooth in his house. When he enters it, he ends up taking a journey through the lands of Dictionopolis and Digitopolis, and he discovers the true value of learning. 
This story makes a nice discussion point for why we need to learn the skills of logic and problem solving. There are also mentions of angles, subtraction, fractions, word problems and infinity, as well as a character called the Dodecahedron who has one face on each of his twelve faces!