4 ideas
In 1930s Milan, a banker is found shot dead in the flat of Commissioner Carlo De Vincenzi's old friend, a man financially entangled with the victim, which makes the friend the obvious suspect. De Vincenzi must run the investigation himself, so his personal loyalty and his official duty point in opposite directions from the first page.
De Vincenzi treats physical evidence as secondary and asks what kind of person could have committed this particular act in this particular way. Each suspect's temperament, fears and self-deceptions are tested against the psychological signature of the crime. Whoever's inner life fits the act is the culprit, even if the material clues first point elsewhere.
Debt, opportunity and location can converge so neatly on one person that the pattern itself should raise suspicion. The investigator's first task is to separate what circumstances make someone look like and what that person is actually capable of doing. Accepting the convenient suspect means letting the scene's arrangement do the thinking.
Under Fascism, the regime wanted crime shown as rare and Italian society shown as orderly, and it later restricted detective fiction. A restrained, clue-based mystery set among Milan's bourgeoisie could still show greed, debt and moral rot beneath respectable surfaces without making an overt political statement. Read this way, the book's quietness is a survival strategy, and that strategy did not protect its author, who died in 1944 after a beating by Fascists.