Nothing interests Milo. Milo has a ton of toys and books in his room. Milo's parents are not really mentioned in the book, but they sure do seem to buy him a lot of things. But Milo is always wanting more. However, when he gets it, he very quickly doesn't want it anymore. Milo is never satisfied.
Then one day, a huge package appears in his room. Who left it? Milo opens the package, and inside it, there is some kind of D.I.Y. building kit. Milo decides to build it because, he figures, there are no other things worth doing. (Well, actually there are, but he doesn't want to do any of them.)
He finishes building the mystery item, and it turns out to be some kind of large... tollbooth. Milo drives through in his toy electric automobile, because he has nothing better to do. On the other side there is a strange new land, with places such as Digitopolis (the city of numbers), Dictionopolis (the city of words), the "Island of Conclusions" (you get there by jumping), and many more.
Milo meets a watchdog named Tock, the Humbug (a very unpleasant bug who always is saying the wrong thing at the wrong time), and people like the Mathematician (ruler of Digitopolis) and Azaz the Unabridged (king of Dictionopolis). Not to mention going to other places such as the Mountains of Ignorance and the Valley of Sound (which at the moment is completely silent).
During this adventure Milo also must rescue the two princesses Rhyme and Reason, for they have been banished to the Castle in the Air, located in the Mountains of Ignorance, which is swarming with demons. It is in Dictionopolis where Milo first hears about Rhyme and Reason, who were able to solve all problems in ways that left everyone happy. Milo becomes interested in this quest -- it is something interesting to do!
By the end of the book, he realizes that he doesn't have to be unsatisfied all the time. There is much more to do in life than just sitting in his chair and waiting for the day to end. His quest ended, the Phantom Tollbooth disappears, as mysteriously as it appears. In its place, only a note, that begins:
Dear Milo: You have now completed your trip, courtesy of the Phantom Tollbooth. We trust that everything has been satisfactory and hope that you understand why we had to come and collect it. You see, there are so many other boys and girls waiting to use it too...
The Phantom Tollbooth has nothing to do with phantoms. I'm pretty sure that some kids might get confused if I didn't put that out there. This is a great book, a lot like The Lost Track of Time, which I recently did a post on. Anyway, this book must've been coated in a special kind of glue, because It was very, very hard for me to put it down, and I hope that it's the same for you too!
Daddy's afterthoughts: Originally, Julia had something in her conclusion that made reference to Doctor Who, suggesting that maybe this author was inspired by the TARDIS. However, this book was written in 1961, and Doctor Who started in 1963. But she's not altogether wrong, in that the "gateway to another world" is one of the all-time classic tropes in literature, cinema, and TV.
Let's see: The Wizard of Oz, Where the Wild Things Are, Stephen Donaldson's first and second Chronicles of Thomas Covenant the Unbeliever, The Chronicles of Narnia, Alice in Wonderland, the Magic Treehouse books (and the scores of young reader series like it)... and of course, Julia's most recent blog offering, Paige Britt's The Lost Track of Time.
But just because a trope is done a lot does not mean we should not appreciate it when it is done well. This is a classic book that all readers, 8-80, can enjoy.