Great swathes of rock music since the nine­teen-six­ties would nev­er have exist­ed, we’re some­times told, were it not for the record­ings of Robert John­son. Cer­tain­ly the likes of Kei­th Richards, Eric Clap­ton, Robert Plant, and Bob Dylan have nev­er hes­i­tat­ed to acknowl­edge his influ­ence. “From the first note the vibra­tions from the loud­speak­er made my hair stand up,” Dylan writes in his auto­bi­og­ra­phy of his first encounter with John­son’s music. “The stab­bing sounds from the gui­tar could almost break a win­dow. When John­son start­ed singing, he seemed like a guy who could have sprung from the head of Zeus in full armor. I imme­di­ate­ly dif­fer­en­ti­at­ed between him and any­one else I had ever heard.” Not bad for a record­ing old­er than Dylan him­self. In the ear­ly nine­teen-six­ties, the blues as John­son played it seems to have sound­ed elec­tri­fy­ing­ly rev­e­la­to­ry to the gen­er­a­tion of then-young musi­cians who man­aged to hear it, regard­less of their own ori­gins. All such record­ings date from 1936 or 1937, the fruits of just two ses­sions in makeshift Texas stu­dios over­seen by pro­duc­er …