Ordeal by Innocence (1958)

A story of a different change of pace is Ordeal by Innocence (Christie had said that this was one of her favorite books). It's a change of pace because of the situation: the murder was a year ago, the accused had died in prison, and the whole basis of the plot was an unusual one (amnesia). This is one of the "murder in retrospect" mysteries. Others in this same vein are Five Little Pigs, Sparkling Cyanide, Elephants Can Remember, and Sleeping Murder.

The story starts with a geophysicist who visits a family and tells them that their brother, who had died in prison, did not murder his mother. This is a year after the murder, because Dr. Calgary--the geophysicist--had amnesia and couldn't clear the alibi of the accused because he was a witness. This new news to the family is unsettling: if the brother didn't kill his own mother, who did? Something that disturbs Calgary is that the family doesn't care who did! Suspicions and fear enter the household, which leads to another murder. This reveals another plot of fortune hunting and the deceit that can occur in schemes. Novel is well written, but clues are sparse at first, but this is one of the most psychological (there's that word again) stories of Agatha Christie.