Prior to being excommunicated the first of several times, Holy Roman Emperor Frederick II had planned to go on a crusade. However, an epidemic waylaid him and a large part of his army, delaying the conquest. As a result, he was excommunicated by Pope Gregory IX. He eventually went on the postponed crusade and became king of Jerusalem. With Italy as the center of his power, Frederick, a religious skeptic, was in frequent conflict with the papacy. Frederick wrote the first book on what subject? Discuss
Source: The Free Dictionary