Maigret and the Informer

In the penultimate novel in the series Maigret investigates a crime in Paris's seedy red light district

The body of a well-known Parisian restaurateur turns up on Avenue Junot in Montmartre, seemingly having been killed elsewhere. Inspector Maigret is on the case, and soon discovers that the murder may be gang-related after a colleague working in the red-light district receives a tip from an anonymous informer.