It’s no secret that Ryan Adams cranks out songs at superhuman speed. He’s released a dozen albums since his solo debut a decade ago. III/IV, a two-disc collection of 21 leftovers from 2007’s Easy Tiger sessions, doesn’t quite hold together as well as Adams’ best albums, but for a rather lengthy set of outtakes, it’s surprisingly high-quality. Unlike the laid back, singer-songwriter exercise Easy Tiger, most of III/IV’s songs turn up the volume and swerve into post-punk and classic-rock territories. A few songs sound like fragments or variations on one another, which is to be expected (apparently III/IV was conceived as a concept album). But the best tracks — “Breakdown Into the Resolve,” “Stop Playing With My Heart,” “Star Wars” — wouldn’t sound out of place on Adams’ most recent albums with his scrappy band the Cardinals. He just got clean when he recorded these songs, and their energy is addictive.