Originally released as a contractual obligation, Sleep Dirt is one of Zappa’s overlooked gems. Bookended by lengthy guitar workouts, it’s a nice showcase for his development as a soloist. But the real meat’s inside: on a handful of songs featuring his 1974 group (“Time Is Money,” “Regyption Strut”) Zappa’s band shines. Songs are fully composed and layered with overdubs, but loose enough for the band to breathe and take short solos. And don’t sleep on the title track, a rare Zappa excursion on acoustic guitar. If you can, track down either the original vinyl or the 2012 CD: a re-release in the 1990s added unnecessary vocals and other effects.