Book by Norton Juster. Milo is a bored child... for him, everything sucks until a purple tollbooth arrives mysteriously.
Milo drives through in his toy car, and discovers a magical land. He is befriended by Tock the dog (and timekeeper), meets a humbug and discovers it's hard to swim back from conclusions.
This is important reading for budding SciFi fans and Illuminati, along with Madeleine L'Engle's Wrinkle in Time.