For almost five millennia, in every culture and in every major religion, indigo-a blue pigment obtained from the small green leaf of a parasitic shrub through a complex process that even scientists still regard as mysterious-has been at the center of turbulent human encounters. Indigo is the story of this precious dye and its ancient heritage: its relationship to slavery as the "hidden half" of the transatlantic slave trade, its profound influence on fashion, and its spiritual significance, which is little recognized but no less alive today. It is an untold story, brimming with rich, electrifying tales of those who shaped the course of colonial history and a world economy. But Indigo is also the story of a personal quest: Catherine McKinley is the descendant of a clan of Scots who wore indigo tartan as their virile armor; the kin of several generations of Jewish "rag traders"; the maternal granddaughter of a Massachusetts textile factory owner; and the...