Novels

Too many timelines

TO CLARIFY: NONE OF THE NOVELS ARE CANON

The shows and movies are, for the most part, set in the Prime timeline. Exceptions are the Mirror universe episodes of TOS, DS9, ENT, DIS; and the Kelvin-timeline movies.

The shows tend to ignore the novels entirely, i.e. not mention them or their events, but usually we have no evidence they don't happen in the prime timeline. Key word being 'usually':

So many little continuity errors exist, and so many massive ones as well, especially in the 70s and 80s books. Things like Spock Must Die where the Organians sentence the Klingons to 1000 years without warp drive. But one era that I've talked about with other like-minded fans before is the gap between TOS and TMP. Before the Star Trek Chronology, many fans and writers may have assumed there was a bigger gap between the two, rather than such a large gap between TMP and TWOK. I've theorized with others before about the option of there being a divergent timeline in this area of history where Kirk has two five year missions (or just an undefined extended one) pre-TMP.

Ryan of Star Trek Litverse Reading Guide, pers. comm., edited for clarity/brevity

And because of Star Trek: Picard (and less so Prodigy), which both have their own associated novels, some of the novels have to be shoved into a branch of the novel timeline so there are no clashes. This is achieved with the Coda trilogy, which retroactively puts all the novels between First Contact and the 2387 Romulan Supernova into what is referred to in-universe as the First Splinter.

Cross-references have a slight color background.