To mark the 20th anniversary of her remarkable 1996 debut, Revival, Gillian Welch and perennial collaborator Dave Rawlings sifted through their vault. The dusty Americana tracks here are deeply rewarding: There are strikingly tweaked alternate versions of album cuts; raw, revelatory demos that have the faint whiff of unearthed front-porch recordings; and a trove of contemporaneous covers, jams, and other gems. Collectively, they offer insight into Welch’s evolution as a singer, songwriter, and performer, and into her invaluable partnership with Rawlings.