(written from a Production point of view)
These Are The Voyages: TOS is a three-volume reference book series that chronicles the production history of Star Trek: The Original Series. Each volume corresponds with one of the three seasons produced of the classic television series and is written by Marc Cushman and Susan Osborn, and published by Jacobs Brown Press.
Nearly three decades in the making, Cushman wrote his magnus opus with the full blessing of Star Trek creator Gene Roddenberry and received substantial input from Original Series Producer Robert Justman after Roddenberry had passed away, until Justman himself passed away. Ultimately, with publication started in 2013 and ended in 2015, the finalized reference work consisted of over 2,100 pages of copy. Yet, and despite the fact that Cushman had actively sought out official endorsement,  the work itself was neither sanctioned nor licensed by the official franchise, but legal nevertheless, as it was published under the "works of academic/journalistic nature" exemption clause as specified in US copyright laws. That fact is emphatically stated in a disclaimer on the back covers of the three tomes, which reads,
- "This book is a work of journalism, protected under the First Amendment, and is not endorsed, sponsored, or affiliated with CBS Studios Inc. or the "Star Trek®" franchise. The Star Trek® trademarks, logos, and related names are owned by CBS Studios Inc., and are used under "fair use" guidelines."
Under normal circumstances, this meant that unlicensed works making use of the exemption clause can not legally feature any Star Trek copyrighted imagery, but Cushman's works however are profusely illustrated throughout with black and white publicity shots and behind-the-scenes imagery. He and his publisher were able to do so, as in the specific case of the Original Series a rather unique situation applied, according to editorial staffer, Archivist/Photo Editor Gerald Gurian. When Paramount Pictures acquired Desilu Studios in 1967, the new owner chose to copyright only the footage from the Original Series episodes that were broadcast on television and not the out-takes, behind-the-scenes material and publicity photos; which, being left unregistered, were thus allowed to become public domain material according to copyright laws, thereby becoming "(...)perfectly appropriate for any publisher to reprint them," as per Gurian. 
This, in current understanding, peculiar circumstance was primarily due to two factors; firstly, at that period in time, Hollywood in general, but television in particular, did not yet recognize the added value of legally claiming material of this nature, especially since a home media format market simply did not yet exist at that time. But secondly, and this was specifically the case with Star Trek, the new owner was not in the least interested in their new property, and was actually looking for ways to cancel the series as soon as possible at the time. It has enabled Roddenberry to procure this material and to widely disseminate it to the public at large in the late 1960s and 1970s through his merchandising company Lincoln Enterprises. It is this material that is featured in the works of Cushman and he did adhere to copyright law by not including any copyrighted imagery from the episodes themselves. It is for these reasons that the contemporary and similarly conceived reference book, Return to Tomorrow - The Filming of Star Trek: The Motion Picture, has not been able to feature any imagery.
Gurian himself started in 2016 with the release of the pictorial To Boldly Go: Rare Photos from the TOS Soundstage full color photo series, specifically intended as companion tomes to Cushman's works.
- These Are the Voyages: TOS Season One, August 2013
- These Are the Voyages: TOS Season Two, 24 March 2014
- These Are the Voyages: TOS Season Three, 1 February 2015
- These Are the Voyages: TOS - official site