How did Agatha Christie, an otherwise unremarkable woman, with no formal education, and no family background in writing produce the biggest-selling books in history? How was this self-effacing woman able to set a standard in detective fiction that has never been surpassed, or indeed, equalled? Alone of her crime-writing contemporaries how was she able to turn a simple and formulaic entertainment into an international pastime for, seemingly, all time?

In both her own books and via my two books about her Notebooks, with their lists of her ‘Unused Ideas’, we can appreciate how her fund of ideas seems to have been inexhaustible; it was, in all likelihood, the envy of her fellow-writers. Throughout her life she was able to rattle off plot ideas and devices with enviable ease. And she could adopt and adapt an earlier idea in such a well-disguised fashion as to render it unrecognisable.

We do know where she got some of her characters and backgrounds. Poirot was given the nationality, Belgian, of WWI refugees arriving in 1916 Torquay and Miss Marple shared some characteristics with Christie’s grandmother. The Boyntons in Appointment with Death were inspired by fellow-passengers on a Nile cruise; Major Belcher, Archie’s boss with whom they travelled on a round-the-world trade mission in 1922, became Sir Eustace in The Man in the Brown Suit; a colleague from her dispensing days inspired a character in The Pale Horse, forty years later. And personal experience often provided a setting – the geography of Burgh Island, off the Devon coast, appears unchanged in Evil under the Sun; the swimming-pool of the actor Francis L. Sullivan (a West End Poirot) provided the murder scene in The Hollow; her own home and garden at Greenway is the setting for Dead Man’s Folly and Five Little Pigs. The Orient Express, an archaeological dig, a sea voyage, a Caribbean holiday – all these personally-experienced backgrounds were put to good (or should that be bad?) use over a half-century of writing. We even know the origins of some plots: the device underlying Lord Edgware Dies was stimulated by a performance by the American actress Ruth Draper; The Mysterious Mr. Quin was inspired by the Harlequin figures from her childhood home, The Mousetrap had its origins in a tragic true-life crime and Death Comes as the End was written at the instigation of Stephen Glanville, a colleague of her husband Max Mallowan. Her knowledge of poisons from her WWI dispensing experience provided her with many plot ideas, well-loved children’s nursery rhymes lent themselves to sinister interpretations and a love throughout her life of crosswords and bridge no doubt stimulated her ‘little grey cells’.

In Agatha Christie’s Secret Notebooks I speculated about her continuing appeal and offered possible reasons – readability, plotting, fairness and productivity.  And I still believe that they are of paramount importance in explaining her worldwide and enduring popularity. But I would add another factor and this may be the most important of all – simplicity. Although at first glance an Agatha Christie murder mystery may seem complicated, her last-chapter explanation always shows the underlying answer to be straightforward. When one simple fact is grasped, all the other pieces of the puzzle click neatly into place. But these simple facts of everyday life that provided many of her ingenious ideas and which Agatha Christie turned into an annual bestseller are observable to all. Everybody knows that…

…some names can be male or female, that nicknames and diminutives can be misleading, or that foreign names often use foreign alphabets…

Or

…that the best place to hide a murder is in a series of murders, or immediately after a natural death, or after an earlier ‘rehearsal’ murder…

Or

…that alibis no longer exist if Body A is identified as Body B and vice versa, or if a live body pretends to be a dead body, or if a killer is not acting alone…

Or

…that narrators are not always reliable, that policemen and children are not always innocent and that ‘foreigners’ are not necessarily guilty…

Or

…that mirrors reverse images, that suicide can be disguised as murder and that sometimes the most obvious person is the villain after all…

None of these facts depend on expert knowledge or an expensive education; they are all self-evident truths known to everyone. And it is on foundations such as these that, for over half-a-century, Agatha Christie built her clever structures of misdirection. Readers happily acknowledged her superiority and were content to let her fool them over and over in her annual Christie for Christmas. As her publishers put it in 1939 on the blurb for Murder is Easy: ‘Surely you won’t let Agatha Christie fool you again. That would be again, wouldn’t it?’ Over 70 years later they are still saying it.