The Phantom Tollbooth by Norton Juster

This ingenious fantasy centers around Milo, a boy bored with life, who comes home to find a large tollbooth sitting in his room. Joining forces with a watchdog named Tock, Milo drives through the tollbooth's gates and begins a memorable journey. He meets such characters as the foolish, yet lovable Humbug, the Mathemagician, and the not-so-wicked "Which," Faintly Macabre, who gives Milo the "impossible" mission of returning two princesses to the Kingdom of Wisdom.