Eifelheim by Michael Flynn Tom is a cliologist, a mathematical historian (Flynn's own term, analogous to Asimov's "psychohistory"), struggling to understand why the German town of Eifelheim, abandoned during the Black Plague, was never resettled. Sharon, Tom's girlfriend and a physicist, drives him out of the apartment to the library so she can complete her own contemplations, which will prove paramount in resolving Tom's questions.
Father Dietrich is a village priest, who, in 1348 discovers, and eventually befriends, a group of travelers from another world who are stranded on Earth, unable to get home.
This is an ambitious book, not merely for Flynn's attempt to unite the concept of extra-terrestrial life and Medieval history, but for his thorough explorations of the Medieval mind, the role of the church in daily life, as well as advanced physics. This is an intelligent, thoughtful novel, less concerned with questions like "What if aliens gave Medieval Europeans advanced technologies?" than deeper questions like, "What if we don't know as much as we think we know, about history, science, or ourselves?"