A propitious phone call from the Brooklyn school where he was a teacher launched a new career for Adam Gidwitz, who was asked to fill in as a substitute librarian for the day. Charged with finding something to read to second graders, he searched his bookshelves and reached for a book he’d never opened: a collection of fairy tales by the Brothers Grimm.

“I decided to read the kids ‘Faithful Johannes,’ about two children who get their heads cut off by their parents—though I did wonder if I could read it to second graders without being fired,” he recalls. “As I read it to them, I saw that some kids were becoming nervous, so I started telling some jokes to make them laugh, and warning them when something bad was going to happen. And when I finished reading, some of them who knew I was trying to write a book came rushing up to me and said, ‘You should make this into a book.’ ”

He was baffled about how to tackle such a project, since, he says, “it obviously already was a book,” but a solution came to him a few weeks later. “I was halfway up the stairs to my apartment,” he explains, “and I suddenly thought, ‘I know how to do this,’ and with my jacket still on, I opened my computer and started to write that Grimm tale exactly as I’d told it to the kids, with the jokes and warnings to calm them down and heighten the tension.”

The result was A Tale Dark & Grimm, published by Dutton in 2011, which Gidwitz followed with In a Glass Grimmly the following year. Both received numerous accolades and became New York Times bestsellers. The author concludes the trilogy with The Grimm Conclusion, due in October with an announced first printing of 100,000 copies. “I’m genuinely excited about this book,” says Gidwitz. “I brought in a lot of themes from the first two books and worked really hard to make it terrifyingly hilarious and fun to read, while trying to address readers’ deeper psychological needs the way the original Grimm fairy tales do.”

Gidwitz will depart from the Grimm world in his next book project, about which he is intentionally vague, noting, “It might draw on the expertise of my wife, who is a professor of medieval history, and on some of the insane folklore of the early Middle Ages. But I can definitely say that it will be with Dutton.”

The author is signing paperback copies of A Tale Dark and Grimm today, 2–3 p.m., at Table 12.