By the late 1930s the influence of black Americans on all American music was evident and much discussed. The swing era was in full force. Gospel was spreading. The major blues recordings were approaching twenty years old, and a blues-inspired brand of hillbilly music had taken hold after the fashion of the "blue yodel" of Jimmie Rodgers. But the music was not pure, according to John Hammond. In order to present the true music of black Americans, Hammond organized a concert at Carnegie Hall in December 1938 to "show both the general public and the serious musician just what it [Negro music] is." Hammond, while acknowledging the prodigious talents of Marian Anderson and Count Basie; among others, wanted to showcase the true folk music of the Carolinas, Georgia, Texas, Arkansas, Mississippi, and Missouri. He and others combed the South for music untouched by...