Indigo Girls are among the most enduring musical outfits to emerge from the late-'80s female singer/songwriter scene and to a lesser degree the later iteration of the Athens, Georgia scene that birthed R.E.M., Love Tractor, and Widespread Panic. The Grammy-winning duo of Amy Ray and Emily Saliers established a devoted national fan base thanks to early hits such as "Closer to Fine" from their self-titled Epic release in 1988; it was the first of six consecutive gold- and/or platinum-certified albums. Their two-women-with-guitars formula may not have been revolutionary on paper,...