Sean Martin "The Cathars: The Rise And Fall Of The Great Heresy" (Pocket Essentials)

This short book explains the origins of the Cathar movement of medieval Languedoc, Italy and the Balkans, placing it in the context of different theological approaches that had grown up in preceding centuries in both eastern and western European Christianity.
Of course it also covers the appallingly violent - and, in some places and at some times near genocidal - and ultimately successful campaign by the Catholic Church and Inquisition to eliminate the Cathars both as a theological current within and opposed to the mainstream church, and to eliminate physically the Perfect, the Cathar equivalent to the priesthood, through mass burnings while giving others the opportunity to recant. The whole experience is a classic illustration of the vast gulf between the Medieval and modern mindset in assuming the measures that are appropriate in even a civilised society to decide which of two (or more) competing views of the world will prevail - a stark and somewhat depressing affirmation of the old adage that "the past is a different country, they do things differently there".
A highly recommended read!