Hard To Be A God by Arkady Strugatsky and Boris Strugatsky

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 ...?