“Americana,” as a musical term, has come to mean music rooted in the folk, blues and country styles that emanated from rural America. But composer Stephen Foster was Americana, too, as was George Gershwin. And so is Brian Wilson, who finally completed this masterpiece, which he originally had slated to follow the Beach Boys’ 1966 album Pet Sounds, until mental illness and group dissention capsized the project. Various songs turned up on later Beach Boys albums—“Heroes and Villains,” “Cabin Essence,” “Surf’s Up” and the No. 1 1966 hit “Good Vibrations”—and some of them were sequenced together on a 1993 boxed set, Good Vibrations, or on bootlegs. This Smile is a new recording; the late Carl Wilson’s voice isn’t present, and some songs are different. Still, it’s 17 tracks of sheer genius.