Eifelheim turns the usual less technologically-advanced culture meets advanced alien culture trope on its head by making the less technologically-advanced culture mediaeval Germans and filtering all those typical advanced SF concepts through a mediaeval mindset and vocabulary. I mean, you have an alien trying to explain advanced multidimensional concepts to the village priest who is talking about God and spirituality. I just thought it was great. Some of the aliens even end up converting to Catholicism!
The present-day parts weren't as interesting, but they do serve to tie the whole book together in what I found to be a satisfactory manner. The mediaeval portion can be a bit grim because this is the era of the Black (or Blue) Death creeping across the land and the aliens are essentially starving to death, but it's still a great book. A reader does need to be willing to accept mediaeval perspectives, though, so if you're not interested in mediaeval historical fiction at all, this may not be your cup of tea.