It's easy to confuse the author of a book with the narrator, especially when that narrator is third person omniscient. Let's break that down. That means that the narrator writes in the third person (calling the characters in the story "he" and "she") and knows everything (that's the omniscient part). Yep. This narrator can tell us what everyone's thinking and doing at all times. Pretty impressive, right?
So the narrator and the author aren't the same person (the author made everything up, the narrator is just relating it to us), but in The Phantom Tollbooth, the narrator sure seems to have control over what goes into the book. Take the introduction of the Dodecahedron:
He was constructed (for that's really the only way to describe him) of a large assortment of lines and angles connected together into one solid many-sided shape – somewhat like a cube that's had all its corners cut off and then had all its corners cut off again. Each of the edges was neatly labeled with a small letter, and each of the angles with a large one. He wore a handsome beret on top, and peering intently from one of his several surfaces was a very serious face. Perhaps if you look at the picture you'll know what I mean. (14.7)
Wait a second, is the author speaking directly to us? He sure is: here, "you" means the reader and "I" means the narrator. Sure enough, there's a picture for us to look at (at least in our copy of the book) just one paragraph later. The fact that our narrator talks about the picture means that he's thinking about the actual format of the book and how it is designed. That's the author's job more than the narrator's, right? Basically, in The Phantom Tollbooth the author and narrator are sort of mashed together. But we like Norton Juster, so that's okay.
What effect does this kind of narrator have on the way we read the book? Why doesn't our author have Milo narrate things as he goes along? Well, think about it this way: if Milo were narrating, we probably wouldn't learn as much as we do. Even in the passage we just quoted, we get some information that Milo probably couldn't have given us (we love the kid, but he just hasn't taken the right math classes yet!). Also, because Milo isn't narrating the experience, we're able to imagine that we are Milo on this trip. Just write "Shmoop" wherever you see "Milo" in The Phantom Tollbooth, and voila! It's us on that magical journey.
One last thing. Why does the author use pictures to help illustrate (pun intended) his point? Well, maybe he's trying to even the playing field: the whole story is told in words, which is kind of unfair to the Mathemagician and his numbers. So throwing a picture in there uses the more neutral art (as opposed to words or numbers). But to be fair, we're glad The Phantom Tollbooth is written in words. Think about it: when the Mathemagician writes a letter using only numbers, King Azaz can't understand it. So if The Phantom Tollbooth were written all in numbers, we'd be out of luck.