Death in Dakota / Dorothy Derr.

Enhanced descriptions from Syndetics:

Tired of her boring marriage to a man who pays no attention to her, Janet Peterson leaves Minneapolis to go back to her small North Dakota hometown. The town she used to think of as deadly dull turns out to be just deadly. An unpopular banker gets murdered, and almost anyone in town could have done it. Was it the widow, the peeping Tom, the teacher, the car dealer, the preacher - or one of the many other people who loathed him? Everyone has secrets, including Janet's boyfriend from twenty-five years before, who - despite a wife, children, and even grandchildren - wants to rekindle his old romance with Janet. Afraid her brother is a prime suspect, Janet teams up with a new friend to find out who the killer is. Before they get to the bottom of it, two more bodies turn up, and Janet finds herself in mortal danger.