Maigret has Scruples, Georges Simenon, 1958 

An unusual Maigret and a very unusual mystery in that the crime and the body only appear in the final pages.  Maigret is approached on morning in his office by a man who complains that he thinks his wife is trying to murder him. The next day, the wife appears.  Though nobody files a complaint and there has been no crime, Maigret can’t let it go and begins the process of trying to understand ‘mental illness’, finally concluding after reading books and consulting physicians that psychiatry is no more than understanding human nature, something that he excels at.  The book concludes with the husband dead, the sister-in-law accused of murder, and the wife off scot free with her 10 million franc life insurance settlement and her lover intact to start the life she has always wanted but did nothing illegal to acquire.  Another fine Simenon.