Hard To Be a God / Трудно быть Богом

"Anton is an undercover operative from future Earth, who travels to an alien world whose culture has not progressed beyond the Middle Ages. Although in possession of far more advanced knowledge than the society around him, he is forbidden to interfere with the natural progress of history. His place is to observe rather than interfere - but can he remain aloof in the face of so much cruelty and injustice...?
The Strugatsky brothers began to collaborate in the early 1950s.
Arkady worked as a technical translator and editor, and Boris was a computer mathematician at Pulkova astronomical observatory.
Their work includes Hard to be a Cod, Definitely Maybe, The Snail on the Slope and Monday Starts on Saturday. Andrei Tarkovsky's much admired film, Stalker, was based on their most famous work, Roadside Picnic."