Third Girl by Agatha Christie

This mystery is clearly one of Christie's later works; it's set in the sixties, England's changing, and Poirot's aging. In fact, one of the characters starts to come to him with a problem and then changes her mind because he's too old. Ariadne Oliver reappears, in fine form. There are some nice bits of mis-recognition in the text, perhaps even too many to make it entirely believable, but in the end it's well worth reading.