But then when the books make references to things that the readers are almost certainly NOT familiar with from the show because they occurred in previous novels (the crew of the Stargazer from Reunion, Worf melding with Spock in some other novel), the writers inexplicably choose NOT to recap these events- in the same books in which we are getting multiple recaps of events from the show!
You're making that a blanket criticism of all of us, and that's not true. I know that I, for one, gave an extensive recap of the relevant events of the prior three TNG novels toward the beginning of
Greater Than the Sum; indeed, I've always felt I recapped too much there. And I do recall
Paths of Disharmony recapping relevant stuff from earlier Andorian-related books and
Rough Beasts of Empire recapping the gist of the Romulan-schism story arc from prior books. (On the other hand, I'm still annoyed with myself for a recap oversight in
Over a Torrent Sea: I mentioned the Caeliar and I mentioned the defeat of the Borg, but I never specifically mentioned the connection between the two.)
As a rule, the goal is to explain
any backstory that is relevant to the story being told, regardless of whether it happened in a prior episode, a prior novel, or some unchronicled event that's being established for the first time. As
Therin said, any book may be a reader's first book, and there's no way of knowing which prior books or episodes the reader may remember. It's just good sense to approach any tale the way you'd approach an original tale, to structure it to stand on its own and explain any relevant information rather than being dependent on familiarity with external works.
But one has to draw a balance between explaining too little and explaining too much. If that backstory isn't relevant to the tale being told, there's no need to mention it. Okay, so there's a book that mentioned Worf melding with Spock but didn't specify when or why. Was it important for that particular story to know when and why? There was a DS9 episode where Worf mentioned having battled Kelvans sometime in the past. The episode never explained that rather startling revelation, because the details weren't relevant to the story being told then and there. All we needed to know was that it was a great challenge he'd surmounted.