Christie's most famous mystery

 In his study of the writer's life published this autumn, Norman uses medical case studies to show that Christie was in the grip of a rare but increasingly acknowledged mental condition known as a 'fugue state', or a period of out-of-body amnesia induced by stress. In effect, the writer was in a kind of trance for several days, he claims.

The mystery, which has puzzled both the police and Christie fans for 80 years, is a why-dunnit, rather than a who-dunnit. It began on the evening of Friday 3 December at Styles, the Berkshire home of the crime writer, by then already an established name, with a sixth novel, The Murder of Roger Ackroyd, selling well. Around 9.45pm, without warning, she drove away from the house, having first gone upstairs to kiss her sleeping daughter, Rosalind. Her abandoned Morris Cowley was later found down a slope at Newlands Corner near Guildford. There was no sign of her.

Such was the speculation that the home secretary of the day, William Joynson-Hicks, put pressure on the police to make faster progress. Even the celebrated crime writers Sir Arthur Conan Doyle, creator of Sherlock Holmes, and Dorothy L Sayers, author of the Lord Peter Wimsey series, were drawn into the puzzle. Conan Doyle, who was interested in the occult, took a discarded glove of Christie's to a medium, while Sayers visited the scene of the disappearance, later using it in the novel Unnatural Death.

Christie was eventually discovered safe, but in circumstances that raised more questions than they answered. Alone, and using an assumed name, she had been living in a spa hotel in Harrogate since the day after her disappearance, even though news of her case had reached as far as the front page of the New York Times. Until now the two most popular theories offered for these strange events have been that either Christie was suffering from memory loss after a car crash, or that she had planned the whole thing to thwart her husband's plans to spend a weekend with his mistress at a house close to where she abandoned her car.