I just finished reading one of the oddest books I've ever encountered (other books on the list include The High House, Canticle for Leibowitz, the books of Charles Williams and George McDonald, Dune, etc). It is also, like the other books on the list, one of the best books I have ever read. The book is Eifelheim, by Michael Flynn.
The book has a rather bizarre premise; it tends to make people laugh, so let's get it out of the way first. The story is set in 14th century Germany, just before the Black Death hits. A group of aliens appears outside a small village, and are befriended by the local priest.
There. Odd, yes? Very. But the whole thing is presented so believably that it almost feels more like a narrative history than science fiction. One reviewer compared it to "Carl Sagan meets Umberto Eco." That's a very good description of the book, though I would argue that it has more of Eco than Sagan. It is not a fluff book: knowledge of medieval philosophy, theology, cosmology, and science is essential (you can get a good feel for those things by reading C.S. Lewis' The Discarded Image). Some knowledge of current physics and dimension theories is also useful, though not necessary.
The story is a very intense, character-driven work. No-one in the story is without serious flaws, but virtue also comes out in unexpected places. One of the things that impressed me the most was that the author took seriously the translation troubles that would occur between the aliens and the Germans. The aliens manage to construct a rudimentary translator, but even then, the reader can see serious miscommunication happening. (this is one problem with most sci-fi/fantasy--no-one takes the language barrier seriously. The animated movie, Atlantis, was particularly execrable in this regard. At least Stargate came up with something remotely believeable for the movie, though there seems to be no barrier in the tv series) However, the author also finds ways for the two peoples to have productive philosophical and scientific discourse, which often drives the story.
I can hardly recommend this book highly enough. Several of my favorite bloggers have also enjyoed the book (Happy Catholic, for one, and I think John C. Wright as well, though I can't find the post. It may have been a different blogger)