Everyone who liked The Phantom Tollbooth should run out and buy A Barrel of Laughs, A Vale of Tears, which is written and illustrated by Jules Feiffer. My mom got it for me when I was about 10 and it was the first book that had me actually rolling around laughing out loud. Before then, I didn't know that novels could do that to me.
The illustrator for this book also wrote a similar novel with a less-serious, more comedic bent: A Barrel of Laughs, A Vale of Tears. Highly recommended as a companion!