In 1976, members of a U.K.-born band, Fleetwood Mac, gathered at Sausalito, California, and Los Angeles to record one of the most exquisite pop-rock albums ever produced, “Rumours.” The group’s lineup, which had evolved since its inception nearly a decade earlier, now included two Americans, a brilliant singer/songwriter/guitarist,  Lindsey Buckingham, and a songwriter who was one of her generation’s most distinctive vocalists,  Stevie Nicks, who would soon launch a successful solo career.