I have met many, many adults who consider this book their favorite book of all time, even though they haven’t read it in twenty or thirty years. And I have met even more adults who have never heard of it.
The story is ultimately an adventure story, with the main character traveling in a strange land, meeting interesting characters along the way, some of whom join him. They stumble upon an epic quest and have to confront many challenges before saving the day and returning home.
But that is just the core plot. The real joy is the metaphor and wordplay. Many concepts are treated literally: Milo physically jumps to the land of Conclusions; The Spelling Bee and Humbug are large talking insects; The princesses Rhyme and Reason have been imprisoned. Milo begins the story bored, but over the course of the tale he learns that motivation and curiosity are the tools you need to help people and solve big problems.
The actor David Hyde Pierce (perhaps best known for the character Niles Crane from the TV show Frasier) recorded an audiobook version that is very well done and a good pick for a long car ride. It sounds trite to say, but it is one that the whole family can enjoy.
There was even an animated movie made in 1970 that captures much of the whimsy of the story, but doesn’t compare to reading the book and imagining the characters and hearing the wordplay in your own head.