ISLAM arose with remarkable speed and mystery. Patricia Crone’s well-stocked mind, clear prose and unflinching intellectual honesty were devoted to explaining why. She had little time for Islam’s own accounts of its origins: “debris” as far as historians were concerned, and hopelessly inconsistent. Far better, she reckoned, to fill the gap with contemporary sources and knowledge of other cultures, from messianic Maoris to Icelanders. That required both personal and intellectual bravery. The central beliefs of Islam, such as the way the Koran took shape, the life of Muhammad and Islam’s relations with other religions, are sensitive subjects. Outside scrutiny can...