I have found the following books to be excellent sources for behind-the-scenes information and insights on the making of the Original Series and the feature films exploits of the TOS crew.
Before purchasing any of the others, a must-have addition to the library of any devoted TOS fan is "The Making of Star Trek" by Gene Roddenberry and Stephen E. Whitfield. (Stephen Whitfield was actually the pen name used by Stephen Poe, who also wrote the behind-the-scenes book of the making of Star Trek: Voyager ... listed later in this article). "The Making of Star Trek" was released while The Original Series was still in production; having a first printing date of September, 1968. Here's a photo of the first printing edition from my library ...
1. The Making of Star Trek, by Stephen E. Whitfield & Gene Roddenberry, Ballantine Books, Sept. 1968
2. Inside Star Trek: The Real Story, by Herbert F. Solow & Robert H. Justman, Pocket Books, June 1996
3. The Art of Star Trek, by Judith and Garfield Reeves-Stevens, Pocket Books, November, 1995
4. Star Trek Creator: The Authorized Biography of Gene Roddenberry, by David Alexander, ROC , June 1994
5. Star Trek Memories, by William Shatner with Chris Kreski, Harper Collins, 1993
6. Star Trek Movies Memories, by William Shatner with Chris Kreski, Harper Collins, 1994
7. The Star Trek Interview Book, by Allan Asherman, Pocket Books, July 1988
8. The World of Star Trek, by David Gerrold, Blue Jay Books, May 1984
9. The Making of Star Trek The Motion Picture, by Susan Sackett & Gene Roddenberry, Wallaby, March 1980
10. Chekov's Enterprise: A Personal Journal of the Making of Star Trek- The Motion Picture, by Walter Koenig, Pocket Books, March 1980
11. The Making of Star Trek: The Wrath of Khan, by Allan Asherman, Pocket Books, October, 1982
12. Captain's Log: William Shatner's Personal Account of the Making of Star Trek V: The Final Frontier, As told by Lisabeth Shatner, Pocket Books, July, 1989
13. Star Trek Phase II: The Lost Series, by Judith and Garfield Reeves-Stevens, Pocket Books, March, 1997
14. A Vision of the Future: Star Trek Voyager, by Stephen Edward Poe, Pocket Books, April, 1998
15. Star Trek Sketchbook - The Original Series, by Herbert F. Solow & Yvonne Fern Solow, Pocket Books, May 1997
The Art of Star Trek has a wealth of outstanding reference photos of Star Trek props and costumes from all of the incarnations of the franchise.
Another fine coffee-table work is:
16. Star Trek: Where No One Has Gone Before - A History In Pictures, by J.M. Dillard, Pocket Books, November, 1994
In addition to "The Making of Star Trek", and "The Art of Star Trek", I have referred to both "Inside Star Trek: The Real Story" (pictured below) and "Star Trek Sketchbook - The Original Series" for many insights.