About a decade ago on a wintry day at a former convent in New Jersey, Rebecca Stead and Wendy Mass took a fateful walk. Both women were attending an annual poetry retreat organized by the writer Susan Campbell Bartoletti. They had met once before but this time they bonded. “We had a mutual admiration thing going,” Mass says. “It gets harder as an adult to make new friends but we clicked in an easy way.”

Wouldn’t it be nice, they agreed, to write a book together, in between their other projects and not on deadline? “We thought it would be a joyful counterpoint to the long lonely slog of writing a novel alone,” Stead recalled.

So sometime later—the authors admit it might have been more than a year—Stead got started. On a flight from Melbourne to Los Angeles, she wrote a scene about 10-year-old Livy, who was returning to Australia, where her grandmother lived, after a five-year absence. As Livy gets closer, she develops a niggling sensation that she had left something there. The chapter ends with Livy opening a closet door and realizing she was right. She did leave something very important behind. Stead emailed the chapter to Mass. “See what you think,” she wrote.

“Rebecca lobbed me a zombie,” Mass recalled. “Or an alien. I wasn’t sure what Bob was. But I decided, ‘I’m going to run with this zombie.’ I had to imagine what he was doing in the closet for five years.”

“I got to have the funny moment,” Stead admits, “and now, over to Wendy!”

Whatever Bob is, he is soon to be published. In February, Jean Feiwel at Feiwel and Friends won an eight-house auction for the North American rights to the first middle-grade collaboration between Newbery Medalist Stead and Mass, who has more than 20 novels under her belt, including the bestselling Candymakers series. Publication is scheduled for May 2018. Stead was represented by Faye Bender of The Book Group; Mass by Ginger Knowlton of Curtis Brown Ltd. For a long time, only their agents knew what they were up to.

“Well, I did tell my kids,” Stead said, “because they are fans of Wendy’s books so it was a little feather in my cap to say, ‘Guess who I’m writing a book with?’ ” But that’s all she told them, and neither writer will say much more about Bob, a short, greenish creature dressed in a chicken suit.

“It was important that we didn’t really know what he was, whether it was a zombie or an alien or what,” Mass said. “A big part of the story is figuring out who he is and what he means to Livy,” because, five years earlier, Livy had promised Bob she would help him find his way back home.

Though Bob’s existence in the closet has been static, Livy has changed. A lot. “Bob has met two Livys, one young and extremely powerful, and now the older Livy who is more skeptical,” Stead said. “What we were most interested in was exploring that growth, what happens to a child between age five and 10.”

Though Stead wrote Livy’s chapters and Mass wrote Bob’s, there is a lot of overlap. Stead asked for advice from a friend, Scholastic’s David Levithan, who has co-written books with John Green and Rachel Cohn. “Both of our characters were in almost every chapter. How do you deal with ownership of characterization?” she asked him. Treat it like they do in improv, he suggested. “Whatever you’re given, you accept it by saying ‘Yes, and…’ That was a fun way to look at it. Learning to value someone else’s imagination was revelatory and so entirely different from writing alone.”

The two writers admit their processes are completely different. Mass says she puts a lot of energy into plotting before she starts. Stead calls herself a wallower. “I create a lot of material, scenes, or things about a character. When I have a bunch of scenes, I roll around in it. This is why I have written four books in the time that Wendy has written 22.”

Mass had to learn to write with a little more open-endedness than she was used to. “I operate on deadlines,” Mass said. “I was always trying to plot things forward.”

“Whereas I was the obstacle to plotting,” Stead confessed. “I had to force myself to move the story along. It can’t be just two people talking about Saturn. That was my challenge. Every time I sent a chapter back to Wendy, in those six to 10 pages, something had to happen. It freed an entire new part of my writing brain.”

Ultimately, despite the differences in how they write, the result was a manuscript whose seams even Mass can no longer see. “Since so much time would pass between when I would work on it, I would always start by reading the book from the beginning,” she said. “It definitely got to the point where I couldn’t remember who wrote what.”

Bob by Rebecca Stead and Wendy Mass. Feiwel and Friends, $16.99, May 2018 ISBN 9781250166623