Back when he released High Top Mountain in 2013, the retro sensibilities of Sturgill Simpson seemed to be rooted solely in outlaw country: he swaggered like the second coming of Waylon Jennings, a man on a mission to restore muscle and drama to country music.

Metamodern Sounds in Country Music, his 2014 sophomore set, was a curve ball revealing just how unorthodox his rulebook was.

After nearly two decades of alternative country doubling down on po-faced authenticity where simpler was better, Simpson embraced indulgence, pushing new wave, psychedelia, and digital-age saturation, all in an attempt to add the cosmic back into American music.

A Sailor's Guide to Earth goes one step further: it's an old-fashioned concept album, one that tells a story -- it's a letter to his newborn son, telling him how to become a man -- and is dressed in garish art suited to the side of a Chevy van.

The overarching aesthetics are a throwback to the golden age of vinyl but Simpson is too smart to succumb to mere revivalism: he seeks to expand, not retract.

To that end, he'll posit that Nirvana's "In Bloom" exists on a continuum that runs back toward Glen Campbell's renditions of Jimmy Webb tunes, which hints at how, for as steeped in the '70s as A Sailor's Guide to Earth is, Simpson doesn't limit his prog to merely rock.

He's equally attracted to the symphonic haze of progressive folk and the boundary-blurring soul of Muscle Shoals, using its thick swathes of horns and smears of slide guitar as binding agents in songs that occasionally need to be pulled together.

Blame that on Simpson sometime prioritizing the journey over the destination.

He's certainly not indifferent to songs -- strong ones punctuate the voyage, ones that veer closer to soul than country -- but he cherishes the voyage, so there are times when A Sailor's Guide to Earth threatens to float away on a slipstream of strings and melodies that are heartfelt and hookless.

Even at these moments, his ambition remains ingratiating: he might not quite arrive precisely where he intended, but as he makes it so clear throughout the album, what matters is the journey itself.