The title of a book is pretty telling of what you'll find inside. So by naming this book The Phantom Tollbooth, our author reminds us that the tollbooth is something we should keep our eye on. Norton Juster doesn't call his story Milo or The Lands Beyond – and he totally could have. But then, we might have focused more on the character or the places he travels than the purpose of the tollbooth itself.
So what is the phantom tollbooth? Well, it's the thing Milo receives in the mail that changes his life by helping him get to the Lands Beyond in the first place. And remember, the story doesn't end when Milo leaves the Lands Beyond: it ends when the tollbooth disappears and moves on to another deserving child. This disappearance is emphasized by the use of "phantom" in the title. Like a ghost, the tollbooth fades away, leaving us to wonder if it was ever there at all. So instead of spending our time unpacking all the puns or figuring out all the allegories, we're able to focus on what the tollbooth teaches us: learning is the key to adventure.
(For more on what tollbooths might mean to our story, check out the section "Symbols, Imagery, Allegory.")