"Gone Girl," by Gillian Flynn (Crown). “Gone Girl,” by Gillian Flynn (Crown).

As millions of readers know, Gillian Flynn’s “Gone Girl” is about a wife’s mysterious disappearance. But this Sunday will mark a milestone reappearance: 52 weeks on The Washington Post bestseller list.

Published June 5, 2012, the book has sold over 3.2 million copies (hardcover and e-book combined).

I asked Flynn, who lives in Chicago, to describe the moment she realized that her third novel was a phenomenal hit.

“That’s an easy moment to remember,” she writes via e-mail. “It was 4th of July last year, and ‘Gone Girl’ hit No. 1. I remember getting the news and laughing out loud. It just had not been in my range of vision. So we got our son to bed, and then went out onto our back porch and popped some cheap champagne and watched all the illegal fireworks crackle off the roofs of neighboring buildings.”

The story, about Nick and his missing wife, Amy, presents a fascinating duet of unreliable narrators. Prodded to speculate about what’s made this novel so successful, Flynn says, “We’re all fascinated with other people’s relationships because we’re always wondering if we’re doing ours right. And we can all look at Nick and Amy and recognize to a (hopefully limited) degree pieces of their marriage: The pride and hubris of courtship, the thrill of meeting one’s match, the domestic calm, the petty squabbles you swore you’d never have, the unexpected power plays.”

Lindsay Sagnette, Flynn’s editor at Crown, agrees that the novel’s focus on a marriage  accounts for its success. “She took something universal and relatable — a relationship — put it under a magnifying glass and turned the creep-factor dial hard to the right,” she writes via e-mail. “People love thinking about themselves and combing through the lives of others on Facebook, so they read the book furiously, forcing friends, family and co-workers to read, as well, so that they could dissect all the terrifyingly familiar moments in the book. I am in no way objective, but damn if Nick and Amy aren’t so well-drawn that at times I felt like they were staring at me from inside the book.”

Sarah Weinman, the editor of “Troubled Daughters, Twisted Wives,” a forthcoming anthology of 20th-century domestic suspense fiction, says the clever structure of “Gone Girl” makes for an irresistible puzzle.

“The mid-novel twist is genuinely surprising, in a way that is likely reminiscent to reader reactions to Agatha Christie’s ‘The Murder of Roger Ackroyd,’ ” Weinman says. “The audacious way Flynn flips expectations and shifts from one portrait of a dissolving marriage to something entirely different still manages to play fair — everything is there in the first half, if you go back and look for it.”

But Weinman notes that “Gone Girl” also reflects our times. “The book is rooted in current-day anxieties, from job loss to culture clashes to . . . the often noxious way we think women are supposed to act at odds with their true selves.”

With a movie version starring Ben Affleck ready to go into production in September, “Gone Girl” probably won’t be gone for a long, long time.