September 26th for the PREQUEL novel? What the hell? Why release a prequel story after the main story? You release it shortly before the movie comes out or at least at the same time as the movie.
Four months after? That's called dropping the ball. I hope that date is wrong.
Just because the story's set beforehand, that doesn't mean it wouldn't still make sense to release after
What if it reveals story details that are meant to be a plot twist in the film?
In fact, the very definition of a prequel is a story that is released later but takes place earlier.
Frankly I'd be surprised if it's anything other than that. I mean, backstory on the prior lives of the Covenant crew is likely not particularly interesting, or at least nowhere near as interesting as what David and Shaw did in between movies.
More to the point, backstory on the crew members is something you'd expect to find in the novelisation of the film rather than the prequel (unless those characters are somehow important to the prequel).