^There are a lot of older novels that are nominally set in the interval between TOS & TMP -- including just about any TOS novel by Howard Weinstein or L.A. Graf -- but few of them really do anything with the period that couldn't have been done in a 5-year-mission novel.
That's kind of my attitude. I periodically flirt with the idea of setting one of my TOS books during the movie era, but if it's just a case of changing the costumes and the ranks and the window dressing a bit, what's the point? Unless I'm planning to deal with Saavik or Gillian or Kirk being an admiral or the Genesis technology or whatever, I might as well set any new standalone adventures during the 5YM.
Sure, in theory, I could have set The Weight of Worlds
or The Rings of Time
or No Time Like the Past
during the movie era, but, whenever I thought about it (and I did), I didn't see anything that I could do in the movie era than I couldn't do during the 5YM.
The exception, of course, being the framing sequence in TO REIGN IN HELL, which had
to be set in the movie era because it followed up on the events of The Wrath of Khan.
In that case, the trip was necessary.