The High Crusade, by Poul Anderson, starts out in 14th century England, where a company of knights and yeomen is forming in the village of Ansby, intent on joining Edward III's campaign in France. They are sidetracked from their goal, however, when an spaceship full of aliens intent on conquering Earth unwittingly lands smack in the middle of them. The Englishmen make short work of the aliens, then the local baron takes the entire village onboard the ship thinking to ride it to France and thence to the Holy Land. Instead the ship takes them to one of the alien colony worlds, with no hope of return. Making the best of things, the English begin a crusade to rid the galaxy of the alien tyranny.

It's short, barely 150 pages. I'd originally read it back in grad school, and although I remembered the premise (really, who could forget it?), I'd forgotten all the details, so it was all new again. It's narrated (using medieval conventions of speech) by a small friar, Brother Parvus, who is central to the plot because he is the most adept with languages of those in the village. He tells the tale of how the galaxy is converted to the feudal system and the Holy Roman Church with sly humor, and indicates when he's making up dialog because he wasn't present for the conversation. It's a fun book. Apparently it was adapted into a movie some years ago with John Rhys-Davies as Brother Parvus (naturally). I'll have to see if I can track down a copy somewhere.