Massacre at Montsegur: A History of the Albigensian Crusade by Zoe Oldenbourg
The crusade was the Catholic Church's response to the rapid growth of a rival Christian religion in the very heart of Christendom - the religion of the Cathars. These heretics drew their strength from the conciousness of belonging to a faith that had never seen eye to eye with Catholicism and was more ancient than resistance movement, so that in the end it was not just the survival of the Cathar faith that was at stake but also that of the Languedoc itself as an autonomous and independent region of France.