Citadel (Languedoc #3) by Kate Mosse

1942, Nazi-occupied France. Sandrine, a spirited and courageous nineteen-year-old, finds herself drawn into a Resistance group in Carcassonne - codenamed 'Citadel' - made up of ordinary women who are prepared to risk everything for what is right. And when she meets Raoul, they discover a shared passion for the cause, for their homeland, and for each other. But in a world where the enemy now lies in every shadow - where neighbour informs on neighbour; where friends disappear without warning and often without trace - love can demand the highest price of all.