God's War: A New History of the Crusades by Christopher Tyerman
The story of how a group of warriors, driven by faith, greed and wanderlust, carved out new Christian-ruled states in the Middle East is one of the most extraordinary of all epics. The crusaders' stunning initial success started a sequence of great Crusades, each with its own story, that fundamentally shaped the Christian and Muslim worlds for two centuries, until the last Crusader castles were finally expunged. The energy and commitment that sent army after army into the eastern Mediterranean also led to the invasion and conversion of Central and Baltic Europe, Spain, Portugal, the destruction of the Cathars in Provence and the settlement of America. Told with great verve and authority, God's War is the definitive account of a fascinating but also horrifying story.