Years ago I read a shared universe trilogy a world in which a great empire had discovered gunpowder just as they were falling to a barbarian invasion.
The first novel took place just as the empire fell to barbarian invaders, and focused on a group of researchers who were just working out how to make cannon work, and who fled to the mountains just as it fell to form what became something like an hidden monastic order to preserve their knowledge.
The second novel occurred centuries later and concerns a disgraced member of the upper classes (descendants of the invading barbarians) who's disgraced, in part, because he's working as an apothecary, and treats the underclass (descendants of the rulers of the former empire), but who still has a grudging place because he is the only apothecary in the city who can cure certain venereal diseases. He ends up fleeing, for some reason, and is helped by the "scientists" from the underground order.
The third novel occurs centuries later yet, as the monastery up in the mountains is dealing with a completely new bunch of barbarian invaders.
One interesting aspect is the magic in the universe, which seemed to be based on a psychic power to make people have accidents. Magicians could, by concentrating on someone, will them to have good luck or to have bad luck.
Would have been late 80's, I think. Three books, three authors. Only the three books in the series.