On a Sunday morning in the spring of 1921, a small boy made a grim discovery as he played on a riverbank in the cotton country of rural Georgia: the bodies of two drowned men, bound together with wire and chain and weighted with a hundred-pound sack of rocks. Within days a third body turned up in another nearby river, and in the weeks that followed, eight others. And with them a deeper horror: all eleven had been kept in virtual slavery before their...