"The Revolutionary Life and Death of the Medieval Cathars." This book, written by Stephen O'Shea, really opened my eyes to the malice of the Church. Pope Innocent III (yes- very ironic) used his power the Great Heresy, to eradicate all non-believers and "heretic Christians" from the map. This book focuses on the Languedoc region of France. The time setting is from 1209-1229.
I think it would benefit many to read this excellent and truthful work.