H. B. Farmer
This book can be downloaded and read in iBooks on your Mac or iOS device.
Disorder reigns in Digitopolis! Something has gone haywire in the the land where numbers rule. Milo, Tock, and the Humbug must return to Digitopolis to restore order to the realm. Their journey begins with a visit to their old friend the Mathemagician, who sends them on their quest. During their adventure they meet a cast of odd characters and encounter some challenging mathematics.
This short story is based on characters from the classic children's book The Phantom Tollbooth by Norton Juster. If you've never read The Phantom Tollbooth, drop what you are doing and read it immediately. Then join Milo, Tock, and the Humbug on their new adventures in Digitopolis.