Knuffle Bunny’s adventures wrap up in Knuffle Bunny Free: An Unexpected Diversion, Mo Willems’s follow-up to Caldecott Honor books Knuffle Bunny: A Cautionary Tale and Knuffle Bunny Too: A Case of Mistaken Identity, which have sold more than 750,000 copies. In the final story, on a trip to Holland Trixie loses and finds her stuffed bunny—and then gives it away to a wailing younger child. Here Willems shares the story behind this picture book.

Where did the story come from? That’s a bit tricky. I’d say that that this story is something that developed over many years. I certainly spent more time on this than I did on the other Knuffle Bunny books. I think the turning point was realizing this was the last book. That sort of drove it in the sense that I wanted to get a sense of completion. I had to throw Moriarty off the cliff.

I knew that Trixie had to take a trip. In the first story, she explores her neighborhood and in the second book she explores something larger—she goes to school and does the transfer of bunnies between two neighborhoods. The story had to expand further in this third book. Of course these books are all loosely autobiographical, and I have family in Holland. As a kid, it was a big thing for me to go there and be in a totally foreign place. Living in a new world, Trixie realizes that she doesn’t have to live with her bunny any more. It is no longer a touchstone for her.

And in a purely visual sense, Holland is a beautiful place. I took the photos for the illustrations, but for the first time I didn’t take every single one. A few of the photos in the spread showing Knuffle Bunny traveling around the world were taken by other people. I took two trips to Holland and drove the length and breadth of the country trying to find things to photograph, which was wonderful. I got incredibly lucky. In one place there was a carnival in town and it was early morning and no one was there. I hadn’t planned to have a carnival in the book, but I pulled my car over and began taking pictures until the security guards got nervous. That’s when I ran!

Why end Knuffle Bunny now? It just feels like it’s time. It’s not up to me. The story kind of dictates the ending for these characters. Ronald Searle says as soon as your children become successful, murder them. These are very personal stories, and I feel as though giving them closure kind of protects them. I don’t think an ending is necessarily a sad or bad thing. Since this is a book about endings, it has to be an end.

And my daughter Trixie, who’s now nine, has had to balance between the real Trixie and the fictional Trixie. The idea of this being the last book with this character may not be a bad thing. She can define herself in a different way.

Knuffle Bunny Free: An Unexpected Diversion by Mo Willems. HarperCollins/Balzer + Bray, $17.99 Sept. ISBN 978-0-06-192957-1; first printing 300,000 copies.