In an interview with the Duke alumni magazine last spring, Mallory said that, as someone who was “very rules-conscious,” he found Patricia Highsmith’s representation of Tom Ripley, across five novels, to be “thrilling and disturbing in equal measure.” He went on, “When you read a Sherlock Holmes story, you know that, by the end, the innocent will be redeemed or rewarded, the guilty will be punished, and justice will be upheld or restored. Highsmith subverts all that. Through some alchemy, she persuades us to root for sociopaths.”

When, in a scene partway through “The Woman in the Window,” Anna Fox thinks about another character, “He could kiss me. He could kill me,” Mallory is alluding to a pivotal moment in “The Talented Mr. Ripley.” On the Italian Riviera, Ripley and Dickie Greenleaf, a dazzling friend who is tiring of Ripley’s company, hire a motorboat and head out to sea. In the boat, Ripley considers that he “could have hit Dickie, sprung on him, or kissed him, or thrown him overboard, and nobody could have seen him.” He then beats him to death with an oar.

Back at Oxford, Mallory has said, he “anointed” Highsmith as the primary subject of his dissertation. But he doesn’t seem to have published any scholarly articles on Highsmith, and it’s not clear how much of a thesis he wrote. An Oxford arts doctorate generally takes at least three or four years; in 2009, midway through his second year, Mallory was signing e-mails, untruthfully, “Dr. Daniel Mallory.” Oxford recently confirmed to me that Mallory never completed the degree.

At Oxford, Mallory became a student-welfare officer. In a guide for New College students, he introduced himself with brio, and invited students to approach him with any issues, “even if it’s on Eurovision night.” According to Tess Gerritsen, who had drinks with him and others in Oxford one night, Mallory mentioned that he was “working on a mystery novel,” which “might have been set in Oxford, the world of the dons.”

Mallory sometimes saw John Kelly, his former professor, for drinks or dinner. “They were very, very merry occasions,” Kelly told me. He recalled that Mallory once declined an invitation to a party, saying that he would be tied up in London, supporting a cancer-related organization. Kelly was struck by Mallory’s public-spiritedness, and by his modesty. “I would have never found out about it, except he wrote to me to say, ‘I’d love to be there, but it’s going to be a long day in London.’ ” (When Kelly learned that I had some doubts about Mallory’s accounts of cancer, he said that he was “astonished.”)

At one point, Kelly noticed that Mallory no longer responded to notes sent to him through Oxford’s internal mail system: he had left the university. Robert Douglas-Fairhurst, his doctoral supervisor, recently said of Mallory, “I’m very sorry that illness interrupted his studies.” Mallory had begun looking for work in London publishing, describing himself as a former editor at Ballantine, not as an assistant. He claimed that he had two Ph.D.s: his Highsmith-related dissertation, from Oxford, and one from the psychology department of an American university, for research into Munchausen syndrome. There’s no evidence that Mallory ever undertook such research. A former colleague recalls Mallory referring to himself as a “double-doctor.”

Toward the end of 2009, he was hired as a mid-level editor at Sphere, a commercial imprint of Little, Brown. In New York, news of this event caused puzzlement: an editor then at Ballantine recalled feeling that Mallory “hadn’t done enough” to earn such a position.

One of Mallory’s London colleagues to whom I spoke at length described publishing as “a soft industry—and much more so in London than in New York.” Hiring standards in London have improved in the past decade, this colleague said, but at the time of Mallory’s hiring “it was much more a case of ‘I like the cut of your jib, you can have a job,’ rather than ‘Have you actually got a Ph.D. from Oxford, and were you an editor at Ballantine?’ ”

Mallory was amusing, well read, and ebullient, and could make a memorable first impression, over lunch, on literary agents and authors. He tended to speak almost without pause. He’d begin with rapturous flattery—he told Louise Penny, the Canadian mystery writer, that he’d read her manuscript three times, once “just for fun”—and then shift to self-regard. He wittily skewered acquaintances and seemed always conscious of his physical allure. He’d say, in passing, that he’d modelled for Guess jeans—“runway only”—or that he’d appeared on the cover of Russian Vogue. He mentioned a friendship with Ricky Martin.

This display was at times professionally effective. In a blog post written after signing with Little, Brown, Penny excitedly described Mallory as a former “Oxford professor of literature.” Referring to the bond between author and editor, she added, “It is such an intimate relationship, there needs to be trust.”

Others found his behavior off-putting; it seemed unsuited to building long-term professional relationships. The London colleague said, “He was so full-on. I thought, My God, what’s going on? It was performative and calculating.” A Little, Brown colleague, who was initially impressed by Mallory, said, “He was not modest, ever.” The colleague noted that many editors got into trouble by disregarding sales and focussing only on books that they loved, adding, “That certainly never happened with him.” Little, Brown authors were often “seduced by Dan” at first but then “became disenchanted” when he was “late with his edits or got someone else to do them.”

Mallory, who had just turned thirty, told colleagues that he was impatient to rise. He found friends in the company’s higher ranks. Having acquired a princeling status, he used it to denigrate colleagues. The London colleague said that Mallory would tell his superiors, “This is a bunch of dullards working for you.” Another colleague said of Mallory, “When he likes you, it’s like the sun shining on you.” But Mallory’s contempt for perceived enemies was disconcertingly sharp. “You don’t want to get on the wrong side of that,” the colleague recalled thinking.

Mallory moved into an apartment in Shoreditch, in East London. He wasn’t seen at publishing parties, and one colleague wondered if his extroversion at lunch meetings served “to disguise crippling shyness” and habits of solitude. On his book tour, Mallory has said that depression “blighted, blotted, and blackened” his adult life. A former colleague of his told me that Mallory seemed to be driven by fears of no longer being seen as a “golden boy.”

In the summer of 2010, Mallory told Little, Brown about a job offer from a London competitor. He was promised a raise and a promotion. A press release announcing Mallory’s elevation described him as “entrepreneurial and a true team player.”

By then, Mallory had made it widely known to co-workers that he had an inoperable brain tumor. He’d survived earlier bouts with cancer, but now a doctor had told him that a tumor would kill him by the age of forty. He seemed to be saying that cancer—already identified and unequivocally fatal—would allow him to live for almost another decade. The claim sounds more like a goblin’s curse than like a prognosis, but Mallory was persuasive; the colleague who was initially supportive of him recently said, with a shake of the head, “Yes, I believed that.”

Some co-workers wept after hearing the news. Mallory told people that he was seeking experimental treatments. He took time off. In Little, Brown’s open-plan office, helium-filled “Get Well” balloons swayed over Mallory’s desk. For a while, he wore a baseball cap, even indoors, which was thought to hide hair loss from chemotherapy. He explained that he hadn’t yet told his parents about his diagnosis, as they were aloof and unaffectionate. Before the office closed for Christmas in 2011, Mallory said that, as his parents had no interest in seeing him, he would instead make an exploratory visit to the facilities of Dignitas, the assisted-death nonprofit based in Switzerland. A Dignitas death occurs in a small house next to a machine-parts factory; there’s no tradition of showing this space to possible future patients. Mallory said that he had found his visit peaceful.

Sources told me that, a few months later, Ursula Mackenzie, then Little, Brown’s C.E.O., attended a dinner where she sat next to the C.E.O. of the publishing house whose job offer had led to Mallory’s promotion. The rival C.E.O. told Mackenzie that there had been no such offer. (Mackenzie declined to comment. The rival C.E.O. did not reply to requests for comment.) When challenged at Little, Brown, Mallory claimed that the rival C.E.O. was lying, in reprisal for Mallory’s having once rejected a sexual proposition.

In August, 2012, Mallory left Little, Brown. The terms of his departure are covered by a nondisclosure agreement. But it’s clear that Little, Brown did not find Mallory’s response about the job offer convincing. “And, once that fell away, then you obviously think, Is he really ill?” the once supportive colleague said. Everything now looked doubtful, “even to the extent of ‘Does his family exist?’ and ‘Is he even called Dan Mallory?’ ”

Mallory was not fired. This fact points to the strength of employee protections in the U.K.—it’s hard to prove the absence of a job offer—but also, perhaps, to a sense of embarrassment and dread. The prospect of Mallory’s public antagonism was evidently alarming: Little, Brown was conscious of the risks of “a fantasist walking around telling lies,” an employee at the company told me. Another source made a joking reference to “The Talented Mr. Ripley”: “He could come at me with an axe. Or an oar.”

In protecting his career, Mallory held the advantage of his own failings: Little, Brown’s reputation would have been harmed by the knowledge that it had hired, and then promoted, a habitual liar. When Mallory left, many of his colleagues were unaware of any unpleasantness. There was even a small, awkward dinner in his honor.