Now that Dr Janet Morgan's authorised biography is published, the puzzle of how and why a reserved, dignified upper-middle-class lady became "the undisputed queen of crime" is more or less solved.

But the enigma of Agatha Christie's life and personality will continue to be as fascinating as her own ingenious murders.

Frances Donnelly follows the clues, with the aid of Dr Janet Morgan, Harry Keating, Christianna Brand and others.