Milton's Areopagitica has attracted the attention of scholars for centuries, some of whom have compiled commentaries on words and passages. An example of this is Sir Richard Webb's Areopagitica (Cambridge UP, 1918), which provides the following commentary on this passage:
P. 5, l. 29. that part which preserves justly every man’s copy to himself.
A book, when licensed, was entered on the register of the Stationers’ Company, with the name of the printer or publisher. It was then the “copy” of the printer or publisher; i.e. he had the copyright. The Order of the Lords and Commons, June 14, 1643, provided against the infringement of such copyright: “And that no person or persons shall hereafter print, or cause to be reprinted any Book or Books, or part of Book, or Books heretofore allowed of and granted to the said Company of Stationers for their relief and maintenance of their poore, without the licence or consent of the Master, Wardens and Assistants of the said Company; Nor any Book or Books lawfully licenced and entred in the Register of the said Company for any particular member thereof, without the licence and consent of the owner or owners thereof. Nor yet import any such Book or Books, or part of Book or Books formerly Printed here, from beyond the Seas, upon paine of forfeiting the same to the Owner, or Owners of the Copies of the said Books, and such further punishment as shall be thought fit.” (Arber’s edn of Areopagitica, p. 27.)
https://oll.libertyfund.org/titles/milton-areopagitica-1644-jebb-ed