Labyrinth by Kate Mosse. A two-time-stream historical novel set partly in thirteenth-century Carcassonne and partly in the present — better written than most of its kind, and built around the Albigensian Crusade. The sequels (Sepulchre, Citadel) continue the south-French setting.
The Yellow Cross by René Weis. A serious history of the last Cathar communities in the Languedoc, drawn from the Inquisition records of Bishop Jacques Fournier of Pamiers. Slower going than Labyrinth but the real story.
Montaillou by Emmanuel Le Roy Ladurie. The classic — a microhistory of a single mountain village under Inquisition surveillance in the early fourteenth century, reconstructed from the same Fournier register. Reads like a novel; if you only read one book on medieval southern France, read this.
Histoire des Cathares by Michel Roquebert. The standard French history, in five volumes — but the one-volume L'Épopée Cathare covers the same ground without the bulk.
The Perfect Heresy by Stephen O'Shea. A readable English-language popular history of the Cathar wars, well suited as background reading before a trip to Carcassonne and the Cathar castles.