This book is all about learning, but sneakily so. On the surface, The Phantom Tollbooth seems like an exciting quest fantasy, with bad guys, sidekicks, and beautiful princesses. But, as Adam Gopnik says, The Phantom Tollbooth's "real subject is education" (source). All the elements of the quest really show off the importance of learning different subjects, and despite what King Azaz and the Mathemagician say, proving that both words and numbers are super-important parts of Wisdom. After each episode in his quest, Milo has discovered another important thing about education. He's learning as he goes along, and by the time he gets home, he's figured out that he can create his own magical quest anytime he wants.
Ultimately, all Milo's experiences in the Lands Beyond show him what his own school couldn't: learning is important and can help him get through whatever obstacles life throws his way.
The fact that words and numbers are so deeply separated in the Lands Beyond raises serious doubts about how Rhyme and Reason will be able to unite them after returning.