I remember someone telling me about the Cathars many years ago (perhaps it was in French class?) but I didn't remember the whole story. I've since Googled the Cathars and learned plenty.
This was quite an enjoyable book, even if I did have to read the ending twice to figure out what happened. I like books which take a historical event and 'fill in the blanks' and tell the story from an ordinary (not necessarily a historical) person's viewpoint.