As Barnard says in A Talent to Deceive, Agatha Christie's characters are one-dimensional, her settings are undistinguished, and her prose and dialogue are nothing special. By comparison, her fellow mystery writers of the 1920s and 1930s, Dorothy L. Sayers, Margery Allingham, Ngaio Marsh and Josephine Tey are much better novelists,
And yet, Christie has far more readers than any of them. In fact, by all accounts, Agatha Christie is the best-selling author of fiction of all time. Only The Bible and Shakespeare have sold more copies than she has.
Barnard gives two ideas about why Christie continues to be so popular. First, she does one thing better than anyone: manipulate the reader into guessing wrong about who committed the murder. Barnard's analysis of how she does so is worth studying.
Second, he suggests, her mysteries should not be read as modern fiction but rather as an older kind of story, the tale. He mentions "Sir Gawain and the Green Knight."
One might also think of fairy tales in which character types---an old man, a wicked woman, a young lover, a trickster---play out the action in a simple setting---a forest, a castle, a village. The tale is told in simple, conversational language.
In the tale, there is no attempt at realism or literary sophistication. The plot is everything.
I don't know if anyone has come up with a better explanation for Christie's phenomenal success since Barnard published this book in 1980, but his ideas have me reading Christie again.