One of the odder Poirot novels, this is a posthumous adaptation of a play starring Poirot that was written by Christie. Christie as playwright is a little more mannered, a little less vicious than she is in novel form.  And so, there's a bit of a spark missing here. The Dupin/Poe reference hangs a little heavy right in the middle of it - would, truly, the audience miss it? Would the characters? It's adequate as an adaptation; just a little misbegotten.