I'd be interested in knowing which ones are the best stories.
Particularly (let's put it this way) those well-written novels that, if the book was an aired episode in the series' original run, would be able to be aired exactly as written (in accordance with 60s TV standards).
I have recently looked at the numbered book corpus using similar criteria, with an eye towards encouraging my preteen son (who loves TOS) to carry over this enthusiasm into reading. Without prying too much, it sounds like perhaps you have tasked yourself with a similar mission. Here's my feedback to date, books having been chosen more or less at random:
Ice Trap: Nothing objectionable, and had some well-executed action sequences. But the story was not particularly strong, and it did not 'feel' like an episode--more like a collection of 'big budget' action sequences that we were meant to get excited by, knowing that they could never have been included in an episode.
The Joy Machine: I had a more favorable opinion than others on this site, but it does feel like an unused 52 min TOS script with a somewhat incongruous middle third shoved in there to pad it out to book length. (Which is precisely what it is). Nothing objectionable, although what is effectively drug addiction is a central part of the plot. (Not 'objectionable,' per se, but might not be what you're looking for).
Web of the Romulans: This is one of those that gets dismissed around here as an early, forgettable Pocket entry (and I see why), but it was actually a great selection for my target audience: relatively short, jumps right into the action, dialogue between the principals seems true to the original characters, even has a 'humorous' (YMMV) subplot. Very much had a TOS episode feel (albeit a mid-tier episode). Nothing objectionable.
Windows on a Lost World: I actually greatly enjoyed this one, although some on this site despise it because the premise is a little bonkers (run a search for all the spoilers you want). The crew interactions seem true, and to her credit the author really went for it in terms of worldbuilding and (truly) alien cultures. Nothing objectionable.
Pawns and Symbols: Highly underrated, largely dismissed because it depicts Klingon culture in a manner that was not consistent with TOS novels of its day, much less what came later. (Who cares?) But there is a ton of (largely 'offscreen') sex in it, some of it 'problematic,' to use an overused term. So probably not what you're looking for.
Bloodthirst: I read this when I was in 5th grade and loved it. Has spooky/scary elements, but nothing worse than TOS itself, and I was such a scaredy cat back then it couldn't have been too bad. I remember feeling as though I were watching a good episode.
Enemy Unseen: Loved this as a kid as well but the synopsis indicates there are some sexual contretemps (must have missed that), so perhaps this might not suit my/your present purposes, we will see.
I hope this helps. Feel free to share your own feedback. I'd be interested to hear.
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I think they are actually generally considered bad books. I have not revisited them as an adult because I'm presuming they would not hold up, and why destroy one of a very limited supply of happy childhood memories? Read at your own risk!
