What is American roots music? Any definition must account for a kaleidoscope of genres from bluegrass to blues, western swing to jazz, soul and gospel to rock and reggae, Cajun to Celtic. It must encompass the work of artists as diverse as Alice Gerard and Alison Kraus, George Thorogood and Sun Ra, Bela Fleck and Clarence "Gatemouth" Brown, the Blake Babies and Billy Strings. What do all these artists and music styles have in common? The answer is a record label born in the wake of the American folk revival and 1960s movement politics, formed around the eclectic tastes and audacious ideals of...