"The Making of Star Trek"... This paperback is considered one of the oldest "reference" books for Trek, and the only one for several years.
I doubt there's anything older, since it was published while the show was on the air. It was also one of the first "making of" books about the TV/film production process, so it was a seminal book for people interested in the industry in general.
He details drafting several premises before choosing "...Tribbles" as the most practical. (At least one of these, "The Protracted Man" has finally been realized for the "fan made" series "Star Trek Phase Two".)
Also, Gerrold adapted his rejected premise "Bandi" as a story in TokyoPop's Star Trek: The Manga
Volume 3. And his rejected 2-parter proposal "Tomorrow Was Yesterday" was the basis for Gerrold's Trek novel The Galactic Whirlpool
from Bantam Books -- and indirectly inspired several of Gerrold's original novels, namely Yesterday's Children
, its expanded version Starhunt
, and its rebooted version Voyage of the Star Wolf
and its two sequels (actually based on an unmade TV series proposal that was a reworking of the Starhunt
premise and characters).
So essentially every one of the rejected story ideas Gerrold wrote about in the book The Trouble With Tribbles
has now ended up being made -- one as a novel, one as a comic, and one as a fan film.