Context:

Tom cites this book as a brilliant account of the Albigensian Crusade, quoting from it multiple times throughout the episode including descriptions of the violence and a story about a ghost appearing from purgatory

Context:

Quoted at the beginning of the episode as a source discussing the Cathars, specifically the quote stating that the standard narrative about the Cathars is not true

Context:

Quoted at the end of the episode as a 'fantastic book, absolutely thrilling work of history' that argues the conventional understanding of the Cathars is not true.