Back in 1963, “Mr. Willowby’s Christmas tree/Came by special delivery” in the beloved holiday picture book Mr. Willowby’s Christmas Tree by Robert Barry. Now, 62 years later, Mr. Willowby is again awaiting a special Christmas tree delivery when a snowdrift gets in the way, sparking a sleigh-inspired adventure in Mr. Willowby’s Head Over Heels Christmas, a never-before-published sequel. The project was discovered among the late author’s papers by his son, John Barry, and is being published by Mr. Willowby’s original publisher, Doubleday Books for Young Readers.

“My father passed away in 2012, and I inherited all his artwork files,” Barry said, noting that he then began the task of putting things in order at his family’s Rhode Island home. One of the first stops was a mysterious “ancient gray wooden filing cabinet” located down a dark hallway that he remembered from his childhood. “I had no idea what was in there,” he recalled. But as he sorted through various manila folders and files containing story ideas and sketches, he discovered an unexpected treasure, “a pretty well-developed version—nearly final text and everything—of what became Mr. Willowby’s Head Over Heels Christmas,” Barry said. There were sketches, drawings, and a full mock-up of the book where his father had pasted in blocks of typewritten text next to the artwork. “It was interesting because I could kind of see how my father’s creative process worked,” Barry noted. “Everything was in that folder, even napkins from airplane flights where he was jotting down notes on the back, corner scraps torn out of a newspaper he wrote on, and all kinds of business cards—long before there were Post-it notes.”

The find was intriguing enough for Barry to bring to the attention of the Doubleday team four or five years ago. “I was in touch with them once in a while about various things and we had generally been kicking around ideas about what we might be able to do with my father’s other artwork, or books of his that were no longer in print,” Barry said. “When I found the file, this became a logical thing to pursue,” he added. “They get 95% of the credit. While I stumbled across this ancient paper, they were the folks who got really excited about it, and said, ‘Oh yeah, let’s do this.’ ”

Barry had every confidence in the creative team that had been publishing the original book, which has never been out of print for all these years. In the story, the top of Mr. Willowby’s too-tall Christmas tree is trimmed and passed along—and successively resized to create smaller and smaller pieces—by a parade of human and animal characters. The book’s focus on kindness and sharing proved popular with readers, and it attracted more attention via a couple of big publicity bumps along the way. It was featured on the children’s TV program Captain Kangaroo shortly after its publication and later adapted as a Muppets holiday special in 1995, in which Robert Downey Jr. played Mr. Willowby. All the while, the title remained a family favorite at the holidays and was even embraced by nature lovers for its environmentalist echoes.

Once plans for the Mr. Willowby sequel became a reality, Barry noted that it was a bonus to have so many of his father’s original materials to work with. “We took his near-finished version of the text, put it together with illustrations in his style, based on the sketches and the storyboards that were in the folder, and that’s how the book came to life,” he said. Author-illustrator Sue DiCicco was selected to create the artwork that would carry Mr. Willowby and other characters from the first book forward into the new one. “We were trading drafts and concepts and rough drawings and things like that,” Barry recalled. “I’m not an artist, but I think I have a pretty good idea of how my father would have thought about things,” he said. “I tried to do him honor, flagging things or making suggestions that I thought were in the spirit of what he would have done.” The collaboration was enjoyable and especially rewarding for Barry. “It worked out great. And it was fun to me because I think if my father was still around, he’d be extremely amazed at how this lost file actually got into print.” Barry also contributed an afterword containing more information about his father’s life and career as well as some of the unearthed storyboards.

The response to Mr. Willowby’s Head Over Heels Christmas so far has been gratifying for Barry. “I read reviews online and people are saying the first book was a favorite that they have gone on to share with their children and grandchildren,” he said. “For those folks, this new book is very much in the spirit of the original. And I hope that maybe it introduces what are now the two books to a new generation of readers. There’s nothing better than having a favorite book that you return to again and again, and that you can read with somebody else.”

Mr. Willowby’s Head Over Heels Christmas by Robert Barry. Doubleday, $18.99 Oct. ISBN 978-0-593-70819-4