Several excellent behind-the-scenes Star Trek books were published during the late 1960s and the 1970s. Being a keen appreciator of television and movie production, I often enjoyed these more than most of the Star Trek fiction that was released.
Let's start with the grand-daddy of all Star Trek behind-the-scenes books -- The Making of Star Trek. It is a fascinating look at the production of the show, written contemporaneously with its creation. The book includes b&w photographs, sketches, memos, budgets, and much more. This was one of my earliest book purchases, probably in the late 1970s. The copyright page references an 18th U.S. printing in August 1976, and a first Canadian printing in October 1968. I bought the book in Canada, but I'm quite sure it's not a 1968 print. Perhaps a typo? Did they mean October 1978? Or perhaps this is the 18th U.S. printing?
Another early insider book is The World of Star Trek by David Gerrold. It was first published in 1973. My copy is from the 10th printing, and was purchased on 19 July 1982. A revised and expanded edition, which includes information on the first three Star Trek films, was published in 1984. My copy is from the first printing and was purchased 11 July 1984.
David Gerrold published another insider guide in 1973, this one focused specifically on The Trouble With Tribbles, his well-received episode from Star Trek's second season. My copy is from the 8th printing, and was purchased on 7 July 1983.
Susan Sackett was Gene Roddenberry's personal executive assistant for many years. She has published several Star Trek books, with Letters to Star Trek, published in 1977, being the first. My copy is from the first printing and was purchased on 17 June 1982.
Walter Koenig wrote a very insightful and well-written behind-the-scenes book entitled Chekov's Enterprise, published in early 1980. It focuses on his experience making the first Star Trek film. My copy is from the first printing and was purchased on 19 July 1982.