This is ( as many people have pointed out ) not an actual Miss Marple book, instead they have taken another book and included Miss Marple in the story. I personally think, who cares? This is the third series of Miss Marple and if you have watched the previous seasons of Miss Marple, you have probably realized they're not that faithful, I don't mind that at all, because they ( and this one is no exception ) are great entertainment.
Leo Argyle was hanged for the murder of his unpleasant mother. Two years later his ex- husband decides to marry another woman, this woman happens to know Miss Marple and invites her over for the wedding. However, this takes a terrible turn when a man comes to their house and gives them evidence that Leo Argyle couldn't commit his mother's murder. This upsets the family and causes rifts between the surviving members, and before Miss Marple can solve the mystery, another murder occurs.
This is a great episode, fine the conclusion is nothing too amazing, even though I didn't guess it. The only other episode of Miss Marple that I have seen so far is Towards Zero, and I personally preferred this one, because every moment was so enjoyable.