I am (slowly, I fear) working my way through the entire published corpus of Star Trek books. There are nearly a thousand–too many for me to recognize trends by looking at them all individually. I will collect my thoughts on some suitable subset of them (generally chronologically) periodically, in hopes of cutting the problem down to a more manageable size.
So, let's begin at the beginning. Early fanfic authors and zine editors say they wanted more Trek, and there wasn't any, so they made their own, and nearly six decades later fans are still at it. Eventually, the suits clued in and started publishing Star Trek novels by the shelf-full (and comics, movies, and everything else–but this is about the books), but at first, there were only James Blish's adaptations of the TV scripts on offer.
There are twelve numbered books, plus one extra:
The final of these books includes an original story featuring Harry Mudd (in addition to adaptations of the two episodes he's in). Beginning with Star Trek 6, the books were actually written by Blish's wife, Judy (J. A. Lawrence), and her mother (Ketterer, 1987, p. 25), with Blish doing only "quality control" (Ayers, 2006, p. 9). I did enjoy the writing more in several of the later books–perhaps that's why.
Looking back over my notes, I'm surprised by how positive I was about them, overall. By the time I got through all of them and Alan Dean Foster's adaptations of Star Trek: The Animated Series too, I was really sick of reading novelizations, and it colored my memories of these.
Which isn't to say they're particularly good! They suffer from being straight adaptations of TV scripts, with very little in the way of added depth. They served as a barely adequate substitute for the episodes, I imagine, in a time before home video or streaming, but I guess at least half of the adaptations were outright bad, usually either because they adapted bad episodes or because they adapted episodes that were carried by the strength of the acting.
To boil it all down to a single recommendation: skip these. There were high points and low points, but you won't miss anything, and your time is better spent on other books.