In the 1930s and 40s, a father-and-son team of folklorists hit the highways, byways and rural routes of the United States, traveling in a battered pickup truck laden with primitive recording equipment. John A. Lomax and his son, Alan, covered thousands of miles, stopping off at tarpaper shacks, juke joints, prison yards, and other out-of-the-way places to listen to native singers and to record them for the Library of Congress archives. The Lomaxes made over 10,000 field recordings, and from this...