'Phantom Tollbooth' is 50.

 Norton Juster wasn't looking to write a children's classic, or even a book. What became The Phantom Tollbooth began as a way of putting off work on the book he was supposed to write: a treatise on "urban perception," or the way people experience modern cities. "I was in over my head," Juster recalled recently from his home in Amherst, Mass. "I was just trying to get the book on cities out of my mind."

Tollbooth, according to the publisher of its newly released 50th anniversary edition, has 4 million copies currently in print. Although the book is Juster's enduring legacy, he graduated from the University of Pennsylvania in 1952 with a degree in architecture, and has spent most of the last 50 years practicing and teaching the discipline.

During his time in Philadelphia, the Brooklyn-born Juster acquired an enduring taste for scrapple via frequent meals at Linton's. He still stocks up at the Reading Terminal Market every time he's in town, as he will be Saturday, when he'll be interviewed onstage at the Free Library's central library by local author Alex Stadler.

It's fitting that The Phantom Tollbooth was conceived while Juster was essentially playing hooky, since the book is driven by the restlessness of a child's imagination. Juster describes his gangly protagonist, Milo, as a boy "who didn't know what to do with himself - not just sometimes, but always. When he was in school, he longed to be out, and when he was out, he longed to be in."

Milo's journey, commenced via passage through a miniature tollbooth that mysteriously appears in his bedroom, stretches between the twin cities of Dictionopolis and Digitopolis, the former ruled by the verbose King Azaz, the latter by the numerically focused Mathemagician. Along the way, he passes through a series of like-minded domains, each presided over by a monomaniacal figure: He meets Chroma, a conductor whose orchestra generates the colors of the rising sun, and the Soundkeeper, a paternalistic tyrant who has decided that since his subjects no longer appreciate the beauty of sound, they must live without it.

From a grown-up perspective, The Phantom Tollbooth's authority figures may seem like academics. Each is determined to view the world exclusively through the lens of his chosen discipline. But for Jules Feiffer, whose lively illustrations have become inseparable from Juster's text, the prevalence of self-appointed (often incomprehensible) experts closely mirrors a child's experience of adults, especially teachers.

"That's what kids feel about the grown-up world," he said, "that you have these countervailing egos that always 'know' something. What they think they know may contradict what everyone else knows, and it may also contradict what you yourself think, but you're so powerless you'd better keep it to yourself."

"I started to think about when I was a kid, and it all flowed from that," Juster, 82, recalled of the book's genesis. "I didn't have a terrific memory for events, but I had a good memory of the way I used to feel about things. I would feel oppressed or I would feel disconnected or I would feel rage - all those things."

What particularly incensed him was to have his imagination lit up by some new idea, only to have the figures who were meant to be helping him learn steer him away from his fascination and onto more well-trodden ground. He cites a favorite exchange from a Peanuts cartoon, in which Lucy and Charlie Brown discuss their first steps at reading poetry. "How am I supposed to know which poems to like?" she asks. He replies: "Somebody tells you."

Since Juster never set out to write a children's book, he gave little thought to his potential audience, especially with regard to what they might or might not understand. The Phantom Tollbooth is chockablock with wordplay and linguistic gymnastics that are bound to fly over the heads of younger readers: A doctor with a fondness for unpleasant sounds is named Kakophonous A. Discord (the "A" stands for "as loud as possible"). In another scene, Milo steps into a car that will move only if he keeps silent, since this particular vehicle "goes without saying."

Leonard Marcus, a children's book historian who wrote the supplemental material for Tollbooth's new annotated edition, contrasts the book with The Cat in the Hat, written four years earlier.

"That was a book deliberately written to a word list that educators came up with," he said. "The idea was to encourage reading by stressing the child as little as possible with words they might stumble over. Norton went in the other direction, but he did it naively, without realizing he might encounter flak from those same educators."

The annotated edition differs from the 50th anniversary edition: Both newly released books include the novel and illustrations, but the former has an opening essay, margin notes, and archival material at the end. The latter includes essays about Tollbooth by popular children's authors such as Mo Willems and Suzanne Collins.

One of Marcus' most intriguing discoveries was that Juster built scenes and even whole chapters up from lists of words and expressions. In Dictionopolis, Milo is forced to literally eat his words (which, being unexceptional, don't taste very good); the numerologically inclined Dodecahedron, whom he meets on the outskirts of Digitopolis, points out that without numbers, you'd never know how high your hopes could reach, or the width of the whole wide world.

Juster credits his fondness for wordplay to his father, a fellow architect and Groucho Marx obsessive who immigrated to the United States from Romania at the age of 6. Although Juster's father spoke perfect, accentless English, one detects a hint of the non-native speaker's fascination with inexplicable idioms. Why, exactly, does one jump to conclusions - and, once having jumped, how might he return? In The Phantom Tollbooth, "Conclusions" turns out to be an island, easily reached but quite tricky to leave.

Even after 50 years, Juster is still hard-pressed, or simply too modest, to account for the book's success. He credits readers with finding meanings in the book he never knew he intended. This stance is of a piece with the book's underlying philosophy that education is a matter of what you learn, rather than what you're taught. Through his travels, Milo finds a way of reconciling the differences between words and numbers, silence and sound, forging his own idiosyncratic path, one that need make sense to no one but himself.

"The phrase that always used to drive me nuts was, 'How can you be right and the rest of the world wrong?' " Juster recalled. "Well, it turns out that was very likely."