Seventies soul icon Bill Withers has a passionate, sandpapery voice and a gift for writing poignant yet catchy songs. In the friendship tribute of “Lean on Me,” the tense downbeat blues of “Who Is He? (And What Is He to You?),” and the summery disco of “Lovely Day,” his easygoing delivery leaves the impression that the songs just flowed out of him. A final valedictory duet with jazz saxophonist Grover Washington, Jr., “Just the Two of Us,” perfectly captures the era’s groovy romance.