Maigret and the Saturday Caller — Georges Simenon

Maigret is visited by a troubled man and asks him to keep in touch, hoping to curtail his criminal impulses -- but when the man disappears, Maigret must investigate a crime that may or may not have occurred

When Maigret returns home at the end of a long day, he is surprised to find a tense man named Léonard Planchon waiting on his doorstep with an unusual problem. Planchon wants to kill his wife, or perhaps his wife and her lover, who for two years have been making him sleep on a cot in the dining room. He has even worked out a plan to hide their bodies in concrete. Uneasy and hoping to stop the man before he goes too far, Maigret must investigate a murder that has not yet been committed and uncover the truth behind this peculiar ménage à trois.

A fast-paced, psychologically astute mystery, Maigret and the Saturday Caller follows the inspector through Montmartre as he patiently questions Parisian partiers, bartenders, and others as to Planchon's whereabouts, stripping away a bit more of the mystery's camouflage with each encounter until they lead him to their shocking conclusion.