In Who Said Coo?, Lulu the pig is kept awake by two prankster pals—Pigeon and Owl—who make coo-ing, whoo-ing, and finally moo-ing sounds outside her cottage. When the sleepless porker shoos them away, their boo-hoos let her know their feelings are hurt, and she invites them inside, where they drink cocoa and snuggle in for a sleepover. Out this month from Simon & Schuster’s Beach Lane imprint, this picture book is written by Deborah Ruddell and illustrated by Robin Luebs, identical twin sisters from Illinois who have each published two prior books, yet have never before collaborated professionally. Here’s how the book was born.

Two years ago, Ruddell, who has penned two books of poetry for children—Today at the Bluebird Café: A Branchful of Birds and A Whiff of Pine, A Hint of Skunk: A Forest of Poems, both McElderry releases—was in her car on the way to hike in the woods when she heard a song on the radio that provided serendipitous inspiration. “I’m pretty sure I misinterpreted the lyrics, but to me they sounded like, “Who said coo,” she explains. “For some reason those words stuck with me, and while I was hiking, I began thinking about the story that became this book. And that’s the rather simpleminded way it came to be!”

When Ruddell sent the first draft of Who Said Coo? to Luebs, she immediately e-mailed her twin back, with the comment, “A monkey could illustrate this!” “Robin may have come to regret that later, since she turned out to be the monkey,” the author quips, adding, “I definitely got the feeling she liked the story.”

That was surely the case. “Debby’s story just popped off the page,” Luebs notes. “The characters were vivid and I immediately saw the visuals—Lulu’s cottage, her world.” The sisters debated whether Lulu would be a child or an animal before deciding on the pig persona. “I love drawing pigs,” Luebs says. “It’s almost impossible to drawn an ugly pig. They’re so expressive, and that worked for Lulu’s personality.”

Before Luebs created the art, Ruddell sent the manuscript to her agent, Steven Malk of Writer’s House, who also represents her twin. He submitted the script to editor Andrea Welch at Beach Lane, proposing Luebs as illustrator.

Welch at the time was still working on Luebs’s debut solo effort, Please Pick Me Up, Mama!, which she edited, and she warmed to the idea of the sisters teaming up for the project. “Robin had done an amazing job on the earlier book, and to me it was a no-brainer to go with the sister collaboration,” she says. “I was immediately taken not only by Debby’s characters, but with the story’s rhyme, rhythm, and repetition, as well as its great message about conflict resolution, which will resonate with adults as well as kids.”

Luebs, who had previously illustrated Raina Moore’s How Do You Say Goodnight? (HarperCollins), says that illustrating her sister’s text was a joy—and a challenge. “I obviously know her so well—I get her and she gets me,” says the illustrator. “I love her writing so much—her words sing—and I wanted to make sure the art lived up to it.”

Ruddell also faced a new challenge with her first picture book. “Given my poetry background, I mostly focused on the rhythm and the language,” she observes. “But until you’ve written a picture book you really can’t imagine how difficult it is to make so few words do so much, especially since the illustrations didn’t exist yet.”

Once Luebs got to work on the art, Ruddell made frequent trips from her home in Peoria, Ill., to her twin’s studio in nearby Urbana to “be a fly on the wall. I got to see the art at every stage, and it was fun and fascinating to see how a picture book gets put together.”

Welch is thrilled with the results of the sisters’ collaboration. “Their work really does seem to have a very similar tone,” notes the editor. “Perhaps it was because of the twin brain thing going on that Robin was instinctively able to get right into the heads of Debby’s characters and run with them. It all seemed to happen organically.”

Currently promoting Who Said Coo? on what Luebs calls “a worldwide tour of central Illinois,” the sisters both remark that working on a book together fulfilled a longtime dream and that they’d welcome the chance to collaborate again. “Every once in a while I look at this book with our two names on the cover, and I can’t believe it actually happened,” says Ruddell. Her twin agrees: “It’s like a fairytale—one that I honestly thought might never come true. And to do another book together would be the best thing ever.”

Who Said Coo? by Deborah Ruddell, illus. by Robin Luebs. S&S/Beach Lane, $16.99 June ISBN 978-1-4169-8510-5