Dan Mallory, a book editor turned novelist, is tall, good-looking, and clever. His novel, “The Woman in the Window,” which was published under a lightly worn pseudonym, A. J. Finn, was the hit psychological thriller of the past year. Like “Gone Girl,” by Gillian Flynn (2012), and “The Girl on the Train,” by Paula Hawkins (2015), each of which has sold millions of copies, Mallory’s novel, published in January, 2018, features an unreliable first-person female narrator, an apparent murder, and a possible psychopath.

Mallory sold the novel in a two-book, two-million-dollar deal. He dedicated it to a man he has described as an ex-boyfriend, and secured a blurb from Stephen King: “One of those rare books that really is unputdownable.” Mallory was profiled in the Times, and the novel was reviewed in this magazine. A Washington Post critic contended that Mallory’s prose “caresses us.” The novel entered the Times best-seller list at No. 1—the first time in twelve years that a début novel had done so. A film adaptation, starring Amy Adams and Gary Oldman, was shot in New York last year. Mallory has said that his second novel is likely to appear in early 2020—coinciding, he hopes, with the Oscar ceremony at which the film of “The Woman in the Window” will be honored. Translation rights have been acquired in more than forty foreign markets.

Mallory can be delightful company. Jonathan Karp, the publisher of Simon & Schuster, recently recalled that Mallory, as a junior colleague in the New York book world, had been “charming, brilliant,” and a “terrific writer of e-mail.” Tess Gerritsen, the crime writer, met Mallory more than a decade ago, when he was an editorial assistant; she remembers him as “a charming young man” who wrote deft jacket copy. Craig Raine, the British poet and academic, told me that Mallory had been a “charming and talented” graduate student at Oxford; there, Mallory had focussed his studies on Patricia Highsmith’s Tom Ripley novels, which are about a charming, brilliant impostor.

Now thirty-nine, Mallory lives in New York, in Chelsea. He spent much of the past year travelling—Spain, Bulgaria, Estonia—for interviews and panel discussions. He repeated entertaining, upbeat remarks about his love of Alfred Hitchcock and French bulldogs. When he made an unscheduled appearance at a gathering of bloggers in São Paulo, he was greeted with pop-star screams.

One evening in September, in Christchurch, New Zealand, Mallory sat down in the bar of the hotel where he and other guests of a literary festival were staying. Tom Scott, an editorial cartoonist and a screenwriter, was struck by Mallory’s self-assurance, which reminded him of Sam Shepard’s representation of Chuck Yeager, the test pilot, in the film “The Right Stuff.” “He came in wearing the same kind of bomber jacket,” Scott said recently, in a fondly teasing tone. “An incredibly good-looking guy. He sat down and plonked one leg over the arm of his chair, and swung that leg casually, and within two minutes he’d mentioned that he had the best-selling novel in the world this year.” Mallory also noted that he’d been paid a million dollars for the movie rights to “The Woman in the Window.” Scott said, “He was enjoying his success so much. It was almost like an outsider looking in on his own success.”

Mallory and Scott later appeared at a festival event that took the form of a lighthearted debate between two teams. The audience was rowdy; Scott recalled that, when it was Mallory’s turn to speak, he flipped the room’s mood. He announced that he was going “off script” to share something personal—for what Scott understood to be the first time. Mallory said that once, in order to alleviate depression, he had undergone electroconvulsive therapy, three times a week, for one or two months. It had “worked,” Mallory noted, adding, “I’m very grateful.” He said that he still had ECT treatments once a year. “You knew he was telling us something that was really true,” Scott recalled. In the room, there was “a huge surge of sympathy.”

Mallory had frequently referred to electroconvulsive therapy before. But, in those instances, he had included it in a list of therapies that he had considered unsatisfactory in the years between 2001, when he graduated from Duke University, and 2015, when he was given a diagnosis of bipolar II disorder, and found relief through medication. In a talk that Mallory gave at a library in Centennial, Colorado, soon after his book’s publication, he said, “I resorted to hypnotherapy, to electroconvulsive therapy, to ketamine therapy, to retail therapy.”

In that talk, as in dozens of appearances, Mallory adopted a tone of witty self-deprecation. (An audience member asked him if he’d considered a career in standup comedy.) But Mallory’s central theme was that, although depression may have caused him to think poorly of himself, he was in fact a tremendous success. “I’ve thrived on both sides of the Atlantic,” he said. “I’m like Adele!” He’d reached a mass readership with a first novel that, he said, had honored E. M. Forster’s exhortation in “Howards End”: “Only connect.” Mallory described himself as a man “of discipline and compassion.”

Mallory also explained that he had come to accept that he was attractive—or “semi-fit to be viewed by the semi-naked eye.” On a trip to China, he had been told so by his “host family.” At a talk two weeks later, he repeated the anecdote but identified the host family as Japanese.

Such storytelling is hardly scandalous. Mallory was taking his first steps as a public figure. Most people have jazzed up an anecdote, and it is a novelist’s job to manipulate an audience.

But in Colorado Mallory went further. He said that, while he was working at an imprint of the publisher Little, Brown, in London, between 2009 and 2012, “The Cuckoo’s Calling,” a thriller submitted pseudonymously by J. K. Rowling, had been published on his recommendation. He said that he had taught at Oxford University, where he had received a doctorate. “You got a problem with that?” he added, to laughter.

Mallory doesn’t have a doctorate from Oxford. Although he may have read Rowling’s manuscript, it was not published on his recommendation. (And he never “worked with” Tina Fey at Little, Brown, as an official biography of Mallory claimed; a representative for Fey recently said that “he was not an editor in any capacity on Tina’s book.”)

Moreover, according to many people who know him, Mallory has a history of imposture, and of duping people with false stories about disease and death. Long before he wrote fiction professionally, Mallory was experimenting with gothic personal fictions, apparently designed to get attention, bring him advancement, or to explain away failings. “Money and power were important to him,” a former publishing colleague told me. “But so was drama, and securing people’s sympathies.”

In 2001, Jeffrey Archer, the British novelist, began a two-year prison sentence for perjury and perverting the course of justice. Nobody has accused Dan Mallory of breaking the law, or of lying under oath, but his behavior has struck many as calculated and extreme. The former colleague said that Mallory was “clever and careful” in his “ruthless” deceptions: “If there was something that he wanted and there was a way he could position himself to get it, he would. If there was a story to tell that would help him, he would tell it.” This doesn’t look like poetic license, ordinary cockiness, or Nabokovian game-playing; nor is it behavior associated with bipolar II disorder.