Christie at her home in Devon, in 1974. Photograph by Lord Snowdon / Camera Press / Retna

They are assembled—maybe eight or nine people—in a small place: a snowbound train, a girls’ school, an English country house. Then—oh no! A body drops. Who did this? And why, and how? Among those gathered, or soon summoned, is a detective, who says that no one should leave, please. He then begins questioning the people concerned, one by one. In the end, he collects all the interested parties and delivers the “revelation”: he names the murderer and the motive and the method. Almost never does the culprit protest. Occasionally, he goes off and commits suicide, but as a rule he confesses (“God rot his soul in Hell! I’m glad I did it!”) and exits quietly, under police escort. Anyone who has ever seen a Charlie Chan movie, or played Clue, or, indeed, read a detective story of the past half century will recognize this scenario, created by Agatha Christie, the so-called Queen of Crime, in the nineteen-twenties.

The detective story was invented by Edgar Allan Poe, though he wrote only four of them before he lost interest. Other writers picked up where he left off, but the first “career” practitioner of the genre who is still important to us today is Arthur Conan Doyle, whose Sherlock Holmes series appeared from 1887 to 1927. By Christie’s time, at least two conventions had been established. First was the detective’s eccentricity. (Holmes, when he is not chasing a criminal, lies on his couch, felled by boredom and cocaine, shooting bullets into the wall of his study.) A second rule was the absolutely central role of ratiocination. The detective, when he is working, shows almost no emotion. What he shows—and what constitutes the main pleasure of the stories—is inductive reasoning.

Christie, who began publishing detective fiction thirty-three years after Conan Doyle, generally followed these rules, but she elaborated on them, creating the scenario described above—the small place, the interrogations, the revelation—and used it, fairly consistently, in sixty-six detective novels published between 1920 and 1976. According to a number of sources, her books, in the approximately forty-five languages they have been translated into, have sold more than two billion copies, making her the most widely read novelist in history. There is also a continuing output of books about Christie. In the past year, we got two more: “Duchess of Death: The Unauthorized Biography of Agatha Christie” (Phoenix; $25.95), by Richard Hack, who has previously written lives of Michael Jackson and J. Edgar Hoover, among others; and “Agatha Christie’s Secret Notebooks: Fifty Years of Mysteries in the Making” (HarperCollins; $25.99), by John Curran, a devout fan. With Christie, then, we are dealing not so much with a literary figure as with a broad cultural phenomenon, like Barbie or the Beatles.

Christie was born in 1890 and grew up in a large house in Torquay, a seaside resort in Devon. Her father, Frederick Miller, had a modest inheritance, and it sufficed. In her 1977 autobiography, published posthumously, Christie describes her father’s day: “He left our house in Torquay every morning and went to his club. He returned, in a cab, for lunch, and in the afternoon went back to the club, played whist all afternoon, and returned to the house in time to dress for dinner.” She adds, “He had no outstanding characteristics.” Her mother, Clara, did have characteristics. She wrote poetry, and she was interested in the soul. During Agatha’s youth, Clara went through Unitarianism, Theosophy, and Zoroastrianism. Agatha adored her, and spent hours poring over her jewelry and ribbons.

When Agatha was a child, she had no companions to speak of. Her sister and brother, Madge and Monty, were more than a decade older. She had no schoolmates, either, because, for the most part, she didn’t go to school. (She taught herself, out of books.) She was paralyzingly shy; even as an adult, she wrote, she could hardly bring herself to enter a shop. Her social world consisted mainly of the family’s three servants. She also communed, for long periods every day, with imaginary companions: kings, kittens, chickens. Enthusiastically morbid, she adored funerals, and often went to put flowers on the grave of her late canary, Kiki. “I had a very happy childhood,” she wrote.

In one respect, it was not happy. When Agatha was five, Frederick was informed that, apparently as a result of mismanagement, there was almost no money left in his estate. He tried to find a job, but, Christie wrote, “like most of his contemporaries”—she means contemporaries of his class—he “was not trained for anything.” He died young (fifty-five) and discouraged. Agatha and her mother soldiered on. Dinner was often rice pudding.

As a young woman, Agatha had no thought of a career. All she wanted was a husband, and when she was twenty-four she got one: the dashing Archie Christie, a member of the Royal Flying Corps. They married just after the First World War began. Archie was then sent off to France; Agatha worked in the dispensary of a makeshift hospital in Torquay. After the war, the couple settled in a London suburb. They had one child, Rosalind. Archie went to work in the City; Agatha began writing novels. It eventually dawned on her that there was something a little wrong with Archie: he was unapologetically self-serving. She quotes him saying, “I hate it when people are ill or unhappy—it sort of spoils everything for me.” As Agatha, in her thirties, lost her youthful looks and became increasingly successful as a writer, he spent more and more time on the golf course.

In 1926, Clara died, plunging her daughter into the kind of sorrow that Archie found so obstructive to his happiness. Agatha moved into her mother’s house, to ready it for sale. Archie visited occasionally. One day, he arrived and told her that he had fallen in love with a woman they knew—Nancy Neele, a good golfer—and that he wanted a divorce. Thereafter, he lived mostly at his club, seeing Neele on weekends. For months, when he was home, Agatha tried to persuade him to change his mind. Then, one night, she got in her car and drove away. It took the police ten days to find her.

What happened, insofar as it could be pieced together later, is that she abandoned her car near a small town in Surrey, about an hour’s drive from home, then took a train to Waterloo Station, in London. There she saw a poster advertising the Hydropathic Hotel, in Harrogate, a spa town in Yorkshire. That night, she travelled to Harrogate, where she checked into the hotel under the name of Theresa Neele. She spent her days reading and shopping and taking walks.

Meanwhile, a manhunt had been launched. The Surrey constabulary, enlarged to five hundred men, combed the downs and dragged the ponds in the area around her abandoned car. When the weekend came, they were joined by a mob of volunteers, plus bloodhounds. Ice-cream venders set up stands to serve the crowd. Most of the major newspapers carried a daily story on the matter. Christie’s fellow-guests at the hotel looked at the photos of her in the papers, but none of them made the connection. Indeed, she later recalled playing bridge with them and discussing the strange case of the missing novelist.