Robert Johnson_Was the Devil at the Crossroads?

Robert Johnson: Was the Devil at the Crossroads? 
One moment he could barely play the guitar. Six months later, Robert Johnson became the most important blues guitarist of all time. How is it possible? Who, or 'What', did he meet at the Crossroads? Dr. Clarke unravels a devilish mystery that won't go away—a  dark, foreboding tale that shaped the blues and rock n' roll.