Good English translations of books by Arkady and Boris Strugatsky are starting to become available; I recommend their science-fiction-cum-medieval-swashbuckler Hard to Be a God. Star Trek fans will immediately recognize this as a "prime directive" story ("We must not interfere in this primitive society, however dysfunctional") but Hard to Be a God was published in the Soviet Union in '64, two years before Trek's five year mission. Moreover, the observers walking around on this feudal planet, wearing mini-cams disguised as jewelry, are Communist utopians, not Federation would-be-colonialists. The observers see themselves as historians, nothing more. Plotwise, let's just say this particular planet's habit of torturing and killing its intellectuals sorely tests our protagonist's restraint. The antagonist, Don Reba, was originally named Rebia, an anagram for a certain Stalinist henchman; the Strugatskys changed it because it was a little too obvious.
Iain M. Banks wrote a similar tale 35 years later, Inversions, with two competing notions of how to go native. Also recommended.
Not necessarily recommended is the 2013 film version of Hard to Be a God, directed by Aleksei German (who died that year). The film mixes Tarkovskian aesthetics with Ubu Roi-ish perversity in a depiction of a completely degraded anti-culture. It's stylistically fascinating but incoherent; the science fiction aspects are subtle to the point of non-existence. For example, the characters are constantly mugging for the camera disguised as a jewel on the protagonist's forehead (which is never explained), yet he is in half the shots, being filmed by we-know-not-what. The book mentions helicopters whisking our agents around the planet; these are not seen in the movie, which is all horses and mud puddles -- like an extended version of the "Bring Out Your Dead" sequence in Monty Python and the Holy Grail.