'They showed that it was important to write well,' says James. Still, all four gave the future writer who made a home in the genre a foundation on which to build. Rather, says James, Christie novels take place 'in Christie-land, which is a very good place to be. She loved reading Christie's books, but never believed that the author's solutions had any relationship to reality. Of the four, James says she was influenced by Sayers' sense of plotting and the quality of her writing. Sayers, Margery Allingham and Ngaio Marsh. In her new book, James lists four authors who wrote during that 'golden age' - all of them female - who helped to ' a rather despised genre into a form which could be taken seriously': Agatha Christie, Dorothy L. And she isn't keeping it under lock and key. James says she never expected to land on the best-seller list - she originally thought it would be 'a wonderful apprenticeship for someone setting out to be a serious writer' - but the longtime fan has amassed a trove of knowledge about the genre. 'It would be a very, very bad detective story at the end if the reader felt, 'Who could possibly have guessed that?' '

'The detective can know nothing which the reader isn't also told,' she insists. James is also particular about the way a plot progresses, especially the manner in which the story's clues may be revealed to its protagonist. We have a detective, who can be amateur or professional, who comes in rather like an avenging deity to solve it. 'We have a closed circle of suspects with means, motive and opportunity for the crime. 'What we have is a central mysterious crime, which is usually murder,' James tells Linda Wertheimer. It's no surprise that she's developed a finely honed definition of what makes for a good detective story.

Her first book featuring the Scotland Yard detective Adam Dalgliesh, Cover Her Face, was published in 1962. James has been writing detective fiction for nearly half a century. James lists some classic examples of the detective-story form.

Dalgliesh cover her face