For the last year, celebrations of hip-hop’s first five decades have attempted to capture the genre in full, but some early stars and scenes all but disappeared long before anyone came looking to fete them. Three excellent books published in recent months take up the task of cataloging hip-hop’s relics, the objects that embody its history, before they slip away.

In the lovingly assembled, thoughtfully arranged “Do Remember! The Golden Era of NYC Hip-Hop Mixtapes,” Evan Auerbach and Daniel Isenberg wisely taxonomize the medium into distinct micro-eras, tracking innovations in form and also content — beginning with live recordings of party performances and D.J. sets and ending with artists using the format to self-distribute and self-promote.

For over a decade, cassettes were the coin of the realm in mixtapes, even after CDs usurped them in popularity: They were mobile, durable and easily duplicated. (More than one D.J. rhapsodizes over the Telex cassette duplicator.)

Each new influential D.J. found a way to push the medium forward — Brucie B talks about personalizing tapes for drug dealers in Harlem; Doo Wop recalls gathering a boatload of exclusive freestyles for his “95 Live” and in one memorable section; Harlem’s DJ S&S details how he secured some of his most coveted unreleased songs, sometimes angering the artists in the process.