There is a stream that courses through American roots music. Its source is in the Appalachian foothills in a place called Maces Springs, Virginia. It was there that A.P. Carter, his wife Sara, and his sister-in-law Maybelle began their careers as three of the earliest stars of country music. These three didn't just play the music emerging from their hill country upbringing. They helped invent it. The stream these three created turned into a rushing river and moved through several generations of musicians, most notably touching the life of one Johnny Cash who first heard the Carters -...