Here's a brilliant piece to read. I agree with it and it brings up a possible angle that we probably haven't given much thought to: http://at-scene-of-crime.blogspot.com/2011/11/rant-against-word-cozy.html
@Griselda - the reason Simon is so obvious is because he is ruled out with his alleged injury. The investigation does not focus on him but on the others, with the exception of Jackie, who also was in someone's presence the entire time that the first murder occurred. She's not cleared of the following two murders, but Simon is. At no time does anyone suggest that he could be the murderer, which is the obvious trickery. We're meant to believe that he did not do it, no questions asked, and that is how the reader knows in Agatha Christie novels who the murderer is. Now, picking up on that, the suspense for the reader lies in how Simon Doyle could possibly have murdered someone.The nail polish was stupid and unnecessary.@ChristieFanForLife - the only time that Christie lets her fans down is with Murder in Mesopotamia. We have the obvious solution of the person who is never once suspected and who also happens to be the spouse, and so the reader can easily guess the murderer. But the suspense of how and why it was done is disappointing. It's not believable. In that case, some other solution would have worked better. The book winds up as being silly.