There are 2 books in the The Unity series. The newest book is Terminus which came out in 2018.

1
Quietus

Book cover of Quietus
In medieval Italy, Niccolucio, a young Florentine Carthusian monk, leads a devout life until the Black Death kills all of his brothers, leaving him alone and filled with doubt. Habidah, an anthropologist from an alien world racked by plague, is overwhelmed by the suffering. Unable to maintain her neutrality, she…
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2
Terminus

Book cover of Terminus
The transdimensional empire, the Unity, has dissolved. Its rulers and agents have been exiled, stranded across a thousand planes of existence. Empires don't die gladly. The living planarship Ways and Means has ended the Black Death ravaging medieval Europe, but it has bigger plans for Earth. Someone is trying to…