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.

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