Georges Simenon's books, as I've noted before, are so thick with atmosphere they almost serve in their own right as guides to the cities in which they're set. He's best on Paris, most notably in the novels dedicated to Chief Inspector Maigret. It would be the work of a lifetime to plot the locations from his books - and many are long gone, of course - but a few obvious sites stand out.
Start at 36 Quai des Orfèvres, the headquarters of the Police Judiciare and Maigret's base, on the Île de la Cité. (This wonderful Life article includes a photo of Simenon climbing the stairs there, in the mould of his hero - and what great ads.) Behind it, on the Place Dauphine, is the setting for the fictional Chez Paul, the bar to which Maigret used to send for beers and sandwiches in the midst of one of his night-long interrogations.
Heading north-east is the Place des Vosges, where both Simenon (from 1924, with wife Tigy) and Maigret lived for a time, and then there's 132 Boulevard Richard Lenoir, as iconic to fans of the French detective as London's 221b Baker Street to admirers of Sherlock Holmes. A market on Thursday and Sunday mornings makes for a characteristic diversion but film-lovers may be tempted to travel a bit further, up to the Canal St Martin, setting for a scene in Jean-Pierre Jeunet's whimsical Amelie (2001), and home to Hôtel du Nord, immortalised by Marcel Carné in 1938. You'll need lunch by now, and one of the many lovely cafés round here should serve just fine.