God's War — Christopher Tyerman

Tyerman's comprehensive history of the Crusades is more interested in the ideology that sustained crusading than in the battles themselves. He examines how the papacy constructed and maintained the theological and legal framework that made holy war legitimate, how different groups — kings, nobles, merchants, the poor — experienced and understood crusading, and how the movement evolved over two centuries from the First Crusade to the failure of the later expeditions.