Black Coffee by Agatha Christie

This mystery was actually written as a play; Charles Osbourne took the play and added the appropriate filler to make it read like a novel after Christie's death. The piece's origin on the stage is obvious, and you can see a lot of the nuts and bolts that are usually more well-hidden in a novel, partly I think to make the theatre-going experience that much better for the viewers. You can tell, also, that the prose isn't all Christie's, but the plot is quite good and more than makes up for it.