There was a good recent thread on this. Much depends on your own preferences.
I posted the image of the first book of the TOS era series Vanguard because I think it would be excellent to adapt to television. It’s about Starbase 47 serving Starfleet in a region of Federation expansion and colonization. It’s somewhat dark and there’s a mystery at the core.
If you’re looking for the Alpha and Omega of the Borg, the Destiny trilogy is excellent.
If you’re into time travel, Christopher L. Bennett has a series of books about the Bureau of Temporal Investigations.
There was also a great anthology of novellas focused on the Starfleet Corps of Engineers.
There are numerous great standalones too.
IcedRaktajino@startrek.website 12 hours ago
I’m currently reading “Star Trek: Destiny” and the Borg seem to be the big bad.