Five Little Pigs by Agatha Christie

It was an open and shut case. All the evidence said Caroline Crale poisoned her philandering husband, a brilliant painter. She was quickly and easily convicted and sentenced to life in prison.
Now, sixteen years later, in a posthumous letter, Mrs. Crale has assured her grown daughter that she was innocent. But instead of setting the young woman's mind at ease, the letter only raises disquieting questions. Did Caroline indeed write the truth? And if she didn't kill her husband, who did?

To find out, the Crale’s daughter asks Hercule Poirot to reopen the case. His investigation takes him deep into the conflicting memories and motivations of the five other people who were with the Crales on the fatal day. With his keen understanding of human psychology, he manages to discover the surprising truth behind the artist's death.