And in the late 1990s, the Houston design firm Pen & Pixel took the hip-hop album cover psychedelic. Pen & Pixel rightly understood hip-hop as a fantasy of abundance and made its covers into conspicuous consumption cornucopias. On its covers, rappers were street superheroes, surrounded by money, cars, jewelry and women in unlikely combinations and proportions.

This was a turning point — away from realism or classical notions of respectability. And it’s noteworthy that this approach came from the South, a region which had largely been maligned by hip-hop’s coastal elites, and therefore had to develop its own hierarchies of taste.

Modern mixtape art, as captured in this book — fast-moving, bug-eyed, wily — is a clear child of Pen & Pixel. The covers are overstuffed, pseudo-lifelike, disorienting. Rappers are holding babies, or weapons, or ice cream, or cleavers. Aliens might be emerging from their stomachs.

This is a far cry from early mixtape art, which was decidedly utilitarian — the fonts were blocky and simple, and the photos were poor quality. Sometimes the track list took up more space on the cover than the photo. Given the rampant bootlegging that was endemic and essential to the spread of mixtapes, the cover artwork might have been photocopied a few times over. These were, above all, D.J. showcases, and most D. J.s were more or less anonymous, a trusted name without a face.

But in the early to mid-2000s, as artists began to use mixtapes as promotional tools, often sidelining the D.J. in the process, branding became newly important. This book skips that period entirely, overlooking essential mixtapes by 50 Cent and G-Unit, and the Diplomats, releases that are as important to those artists’ catalogs as their official albums, if not more so.

Today, some rappers — say, Future or Gucci Mane — take mixtapes that seriously, too. But for the most part, the impact of mixtapes is waning. In the book’s interviews, almost all of the graphic designers indicate that their heyday has likely passed. Creativity has been squeezed out of the business, they say, a victim of diminishing margins and market saturation.

And that’s to say nothing of the recommercialization of the mixtape. Rappers like Drake are now releasing mixtapes straight to iTunes, redefining the form as something other than a steady source of free music. Given that, the freewheeling era in this book already looks like history.