From Robert Johnson to Aretha Franklin, Mahalia Jackson to John Lee Hooker, blues and gospel artists figure heavily in the mythology of twentieth-century culture. The styles in which they sang have proved hugely influential to generations of popular singers, from the wholesale adoptions of singers like Robert Cray or James Brown, to the subtler vocal appropriations of Mariah Carey. Their own music, and how it operates, is not, however, always seen...