Norton Juster, the children’s book author best known for his 1961 classic The Phantom Tollbooth, has died. He was 91.

A representative from Random House confirmed Juster’s death to Rolling Stone. Per The New York Times, Juster’s daughter, Emily Juster, issued a statement saying the cause of death was complications from a recent stroke.

Author, animator, and friend of Juster’s, Mo Willems, posted about his death on Twitter, writing, “My lunch partner, Norton Juster, ran out of stories and passed peacefully last night. Best known for The Phantom Tollbooth and The Dot and the Line, Norton’s greatest work was himself: a tapestry of delightful tales. Miss him.”

Per a biography on the Scholastic website, Juster was born in Brooklyn, New York, on June 2nd, 1929, studied architecture at the University of Pennsylvania, and, after a stint in the Navy, began his career as an architect in New York. With his firm, Juster worked on projects around New England while he also taught architecture and planning at Pratt and Hampshire College.

On top of all this, Juster was a prolific writer, having fallen in love with the craft while in the Navy. In 1961, he published The Phantom Tollbooth, an inventive, pun-packed adventure about a bored boy named Milo who drives his toy car through a mysterious tollbooth and ends up in the Kingdom of Wisdom (the book featured illustrations by Jules Feiffer). The Phantom Tollbooth received rave reviews and soon became a staple of children’s literature. In 1970, the book was turned into a mostly animated movie (there were some live-action sequences), co-directed by Looney Tunes maestro, Chuck Jones; in 2007, it was turned into a stage musical at the Kennedy Center in Washington D.C. (another film adaptation was reportedly in the works as of 2018).

In a testament to its reach and Juster’s writing — and the ideas of logic, reality, illusion, and more that the book plays with — even the counterculture latched onto The Phantom Tollbooth. The 2011 book, The Annotated Phantom Tollbooth, cites a letter from a fan — identifying himself also as Milo — sent to Rolling Stone in October 1970: “If you want to get freaked out of your undernourished head, pick up The Phantom Tollbooth, by Norton Juster. They tell you it’s a kid’s book, but take my word for it, no one who reads it is ever the same. No hype.”

In 1963, Juster followed up The Phantom Tollbooth with another favorite, The Dot and the Line: A Romance in Lower Mathematics (Chuck Jones turned that one into an animated short as well). He continued to publish regularly over the years, with other notable books including 1969’s Stark Naked: A Paranomastic Odyssey, 1982’s As: A Surfeit of Similes, 2005’s The Hello, Goodbye Window, and, what came to be his last book, 2011’s Neville.