Pretty sure Jessica Fletcher was the murderer. Think about it every episode she visits a friend who coincidently gets killed while she's in town. Then the Police for some unknown reason let a book author investigate because they are lazy or incompetent. Then she tells the police who did it backed up by zero forensic evidence snd all wrapped up in under an hour. Does nobody ever notice how she has wrote the book before the murder? Its in the title "murder she wrote".