Book Den has the best explanation/summary of The Phantom Tollbooth that I've ever seen. (hat tip Candlestring via G.K.C.'s Favourite).
I have to admit that I haven't yet enjoyed this as an adult, though many details stick with me from multitudinous childhood readings. John (who never read it as a child) has read it aloud to our children and several of them have read it themselves too. Now I realize how very important it is for me to pull it out again for myself. :)