Good solid Christie, ambiguous clues, layered and conflicting motivations, a puzzling puzzle to watch Poirot untangle with humorous cluelessness from ol' Hastings. The final letter, a look into the thinking of a totally self-involved, what today we'd call a sociopath, adds an interesting end note. In another mood I might even give it five stars, but even if it is a perfect example of the genre, it's no more than that...