The first studio album by David Crosby and Graham Nash since 1976 is rightly dedicated to the late Cass Elliot of the Mamas and the Papas, the hostess queen of hip late-Sixties L.A. who introduced the pair in 1967. She would have approved of the immaculate, high-altitude harmonies glazing this two-CD set and the enduring mutual admiration within. In the real-life rock opera that is Crosby, Stills, Nash and sometimes Young, Crosby and Nash's friendship and vocal empathy have been remarkable constants. There has been a deepening in their eternal-schoolboy tenors but no erosion in their melodic glide and choral precision. In "Lay Me Down" and Nash's "Half Your Angels" (originally a song for the children killed in the Oklahoma City bombing, now loaded with even more unspeakable sorrow), the singers wear the weight of years and derailed expectations with warm poise and no apologies for their vintage folk-pop earnestness. When those voices rise and fall together at the end of the album, in "My Country 'Tis of Thee," it is with elegant optimism and worry —and the perfect harmony you only get from souls joined for life.