There was a second "Files" magazine called The Star Trek That Never Was
. As I recall, both "books" consisted of rejected story pitches or outlines from the extant three seasons of TOS.
The Reeves-Stevens's book Star Trek Phase II: The Lost Series
had a lot of coverage of various scripts & treatments that were worked up in 1977-1978 before the Star Trek Phase II
pilot script was reconfigured as Star Trek: The Motion Picture
. Is that what you remember?
Allan Asherman was, at one point, working on a book also called The Star Trek That Never Was
, to be published by Pocket Books, but it never came out. I remember seeing a cover image in Starlog back in the late '80's. I was really looking forward to that book!