Series of books by Arkady and Boris Strugatsky

Russian

Escape Attempt
Noon 22nd Century
Natural Sciences in the World of Ghosts ( story in Orphia )
Far Rainbow
The Second Invasion from Mars
Wanderers and Travellers ( story in ISF Nov.67, Path Into the Unknown & The Molecular Cafe )
Hard to Be a God

Prisoners of Power - Maxim Kamerrer I
Beetle in the Anthill - Maxim Kamerrer II
The Time Wanderers - Maxim Kamerrer III