Actually the first era of inter-novel continuity began in the mid-'80s, when various novelists began a) doing multiple books with recurring characters and b) cross-referencing concepts or characters from other authors' books. Notable examples include: Margaret Wander Bonanno's Dwellers in the Crucible using John M. Ford's version of Klingons and Diane Duane's version of Romulans/Rihannsu (as well as elements from Vonda McIntyre's portrayal of Deltans in her TWOK novelization, I believe); J. M. Dillard's The Lost Years alluding to the events of Diane Carey's Dreadnought; Diane Duane's Spock's World using Dillard's security chief character Tomson; and Gene DeWeese making his Chain of Attack a loose sequel to Lee Correy's The Abode of Life. Probably the fullest manifestation of this process (which was encouraged by the editor at the time, Dave Stern, I think) was A. C. Crispin's Time for Yesterday -- a sequel to her own Yesterday's Son, but one that incorporated elements from the books of McIntyre, Duane, Carey, Dillard, Ford, Howard Weinstein, Jean Lorrah, and Brad Ferguson. So by the late '80s, most of Pocket's output to date could be considered part of a blanket continuity, even if much of it was incorporated retroactively and some inconsistencies no doubt existed.
This process was scuttled once Roddenberry and Richard Arnold decided they wanted the books to merely follow the show's lead rather than creating their own continuity. So we had a stretch of over a decade with just standalones, and the developing onscreen continuity contradicted more and more of what the early novels did. Still, the internovel continuity that exists today is the second iteration of such a shared-universe approach to Trek Lit.
I spent three weeks with the first book in the trilogy before giving up.
