Norton Juster's book is ostensibly a children's book. However, like much of children's literature, it contains hidden (and not so hidden) aspects that are of delight to adults as well. This, when you think of it, makes sense--the point of children's literature is to educate as well as entertain (one hopes!), therefore, it makes sense that some of the lessons will be more 'adult' than the actual storyline would seem to indicate.
Milo and his various friends and enemies encountered along the way serve to illustrate many of the foibles and quirks of adult life. The Phantom Tollbooth serves as a gateway to a place that embodies the physical manifestations of metaphors.
For instance, in Dictionopolis (a city of words) Milo is invited to a banquet at which one must eat one's words. Just as in our world, sometimes those words can be sour and very hard to swallow.
Also, while you can jump to the Isle of Conclusions, you must reach the mainland again only by swimming through the sea of knowledge. And the water is cold. It is not easy to recover from having jumped to conclusions.
The interplay between concepts, the tension between words and numbers, the divisions and alliances that are made, the enemies who seem to be friends, all of these serve to make a delightful play which will interest children and adults.
Milo, of course, makes it home safely after a fascinating journey, and while he would like to take another trip, the phantom tollbooth is needed elsewhere for other children, too. However, Milo realises that he has his own tollbooth in his imagination, and thus the adventure need never end.