Yeah, the novels will probably be 5YM stories.
But MAM said on the G&T show that his novella is set during The Wrath of Khan:
It's centered around Doctor McCoy, and the bulk of it actually takes place interstitially early during Wrath of Khan; sort of between scenes. There's an entire story that I teased out of a brief sequence at the beginning of the film.
(Thankfully 8of5 has transcribed that part
since I couldn't remember where I heard it at first.)
Not sure how I feel about another Mariotte TOS novel since I wasn't very fond of his The Folded World.
If it has to be more TOS I would rather like to see another Mollmann/Schuster book to be honest. I really liked their A Choice of Catastrophes and would have thought that we would have seen more books by them by now.