Crusaders by Dan Jones (Templers, Magna Carta) tells how a plea from the Byzantine Empire for military aid in its border wars launched a mass movement that upset the co-existence of Christians and Muslims in Palestine. What followed was decades of mayhem and plunder (at various times, Crusaders attacked not only Muslims but Jews and Byzantium itself).  If cutthroat politics and battlefield fireworks catch your interest, this will be a must read.