The High Crusade by Poul Anderson
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A whimsical science fiction story that succeeds primarily due to the seriousness and commitment with which it carries out its central concept. I'll spoil very little by outlining it (but the truly spoiler averse should skip to the last paragraph):
An alien ship touches down in 14th century England just as a local baron is setting off to join the war in France. By dumb luck the Englishmen overcome the crew of the ship and take control of the ship. Hilarity ensues at length.
The concept would probably have been better as a short story or novella, but it works as a short novel solely because the author takes the worldview and ideologies of his characters seriously, and tries to do justice to their impressions. It's not a work of deep psychological observation, but it gives at least a sketch of the furniture of the medieval mind.
Satisfying and fun, if a little bit big for its payload. Recommended for fans of light science fiction or of Poul Anderson generally.