The Name of the Rose by Umberto Eco (1980). Not set in Toulouse, but the Dominican intellectual world it describes — inquisitions, theological dispute, the fate of heretics — is precisely the world that shaped the city's Couvent des Jacobins. Reading it before visiting gives the medieval city a different texture.
Montaillou: The Promised Land of Error by Emmanuel Le Roy Ladurie (1975). A remarkable microhistory of a Cathar village in the Ariège foothills south of Toulouse, reconstructed entirely from Inquisition records. It brings the medieval Languedoc — the landscape, the beliefs, the daily rhythms — to life more vividly than any conventional history. Essential reading for anyone travelling the region.
The Count of Toulouse by Jonathan Sumption (part of his Albigensian Crusade, 1978). The story of the crusade launched from northern France to destroy Catharism in the Languedoc, which nearly destroyed Toulouse in the process. Sumption writes military history with unusual narrative power; this is the best account of the wars that shaped the city.
Suite Française by Irène Némirovsky (2004, posthumous). Two novellas set during the French collapse of 1940, as refugees from Paris flood south toward Toulouse and beyond. Némirovsky was arrested before she could complete the planned five-part cycle; what survives is one of the finest pieces of French fiction written in the 20th century.