Reese Witherspoon and Harlan Coben ‘Fangirled and Fanboyed’ Over Each Other Writing New Thriller Gone Before Goodbye (Exclusive)

For longtime friends Reese Witherspoon and Harlan Coben, writing a book together felt like a natural partnership — even though neither had ever done it before.

Coben, the bestselling author behind 35 books including Tell No One, The Woods and Nobody’s Fool and TV series like Fool Me Once and Missing You has worked in writers’ rooms for the screen, but never considered writing a novel with a partner. That is, until Witherspoon called him.

“I’ve known Reese for about a decade now. And she’s fangirl-ed, I’ve fanboy-ed,” Coben tells PEOPLE. “I’m a big fan of her, she’s read and watched all mine, I’ve read and watched all of hers and been a fan of the work that she’s done for books in general.”

The Academy Award-winning Witherspoon, who founded the Hello Sunshine media company to support and uplift women-centered stories and runs the Reese Witherspoon Book Club, has long been a champion of literature and an avid bookworm. So when she reached out to Coben with a story idea, his interst was piqued.

“She came to my apartment, and I was a little bit wary,” Coben confesses. “I love Reese and I know how smart she is, but I’m not the guy who does the collaborations. And she told me the story, and I was like ‘dang, that’s good, we could do something with that.'”

Right away, the pair started spitballing. “I took out a yellow legal pad, and she and I started to banter back and forth like we had been doing it for years,” he continues. “And two or three hours later, we said we wanted to think about writing this as a novel together, and that’s how it all started.”

As the process went on, they both became “obsessed with the story” that follows a highly skilled Army combat surgeon named Maggie McCabe who finds herself mired in a deadly conspiracy.

“Sometimes you get in a room and it just goes,” Coben says. “Both of us started sending texts at all hours and emails, and we tried meeting whenever we could in person, which is a fair amount, and dove right into it. We were really, obsessed.”

Coben served as the “guy at the keyboard,” because as he puts it, “You can’t ride two horses with one behind.” They went back and forth in a deeply collaborative fashion, because both wanted their voices and perspectives to come through loud and clear. To write a female protagonist like McCabe, Coben said he particularly valued Witherspoon’s perspective.

“There’s a part in the book where she’s sort of describing what she was thinking about as she’s changing and putting on jeans,” Coben explains with a chuckle. “And we were sort of joking that, guys, you know, they just…put on the jeans, I’m all set.”

He emphasized that the partnership between the two was always just that: a twosome. “There’s never anybody else in the room, there was no third party,” he added. “That was one of the things that we both wanted to do right away. This is us, like it or not, warts and all, this is us.”

And since the book hit shelves on Oct. 14, the writers are already hearing from readers that both of them coming across on the page in equal measure. “It’s funny that my friends and Reese’s friends, they all kind of say the same things, like, ‘I see you in this book, but I also see Reese,’ and Reese’s friends are saying the same thing. Which is, you know, exactly what we wanted.”

The other thing they want? For readers to lose sleep over Gone Before Goodbye. “I hope it’s the book you take to bed at 10 or 11 o’clock at night, say ‘I’m only gonna read for 15 minutes,’ and it’s 4 in the morning,” Coben says. “You had to know what was gonna happen.”

Never miss a story — sign up for PEOPLE’s free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer​​, from celebrity news to compelling human interest stories.

Gone Before Goodbye is on shelves now, wherever books are sold.