His title is a bit grandiose, and his subject, the Portuguese discovery and exploitation of Africa and the East Indies in the late 1400s, may appear distasteful. But Nigel Cliff's "Holy War: How Vasco da Gama's Epic Voyages Turned the Tide in a Centuries-Old Clash of Civilizations" is one of the most readable, engaging and provoking books of the season, hands down.