'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.