New Delhi, Aug. 29 -- I love a good Agatha Christie as much as anyone. I still return to the big ones-Murder on the Orient Express, And Then There Were None, Death on the Nile-for their ingenious plotting, impossible murders, and dazzling denouements.

But over the years, I've found myself drawn to another variant of the classic Christie plot: the whydunnit.

Christie may have mastered the whodunnit, but she also loved exploring the why. That led to a fascination with what made people cross the line: jealousy, resentment, greed, class anxiety, and family secrets.

"Agatha Christie always wrote a variety of stories," says Christie historian Mark Aldridge. "Many of her short stories from the 1920s are about the supernatural or human nature,...