Well, I have thought about it and I'm really sorry that I'm unable to recommend you good English sources!

As about the ones in Russian, I'd read "Russia: What Happened to it During XX Century" [1] by Yuri Semenov, which is a theoretical text about the structure of the Soviet society. Sadly, it is not available in English. Someday I will probably translate it myself. I've heard that is was translated to Chinese, but I don't know Chinese, so I can't really tell.

From fiction I'd read "The Gospel of an Executioner" (Russian: "Евангелие от палача") [2] by Georgi and Arkadi Vainers [0] (Russian: Георгий Вайнер и Аркадий Вайнер). I'm not sure if it was translated, but I would certainly search for German and English translations.

Before watching the movie you surely must know something about the Doctors' plot [3], the fact that the rooms and the atmosphere of the main hero's apartment was made after the apartment of Alexey German's father and, probably, that an episode when an old family friend from a foreign country get's punched in a face has had a basis in Alexey German's life. (The idea is that half of his family has fought in the Civil War on one side, and another half on the other. Delivering news from the other half living outside of the Soviet Union could have meant a death sentence for Alexey German's father and misery for all of his family. Even though an Alexey German's pro-monarchism uncle living in exile has urged his old pro-communist Western friend not to visit his brother (and, consequently, an Alexey German's father) during his stay in Moscow, he got drunk and visited him).

I could make some more comments, but I'm not sure if they would do you any good and not spoil anything.

To better understand the Stalin's era overall, I'd read Varlam Shalamov [4], just everything that you can lay your hands on. Tons of his books are translated to German and one to English.

I don't usually read Western literature about the Stalin era because even better books are so full of bullshit. Not that I don't think that Stalin was an evil person, or that the Stalin era and Soviet Union are so mysterious that they are unfathomable to the people in the Western world. It's just that the ideological differences are still huge and the Cold War propaganda in the West IS a form of reality through which, as if it was some sort of glasses, Western people view the stuff that was going on here in USSR. The Soviet lies died when the Union fell, but the Western ones are living and well. Haha, didn't mean to make a rhyme...

Just as a side note: there are some truly great and deep fiction writers that wrote about Stalin's atrocities, such as Varlam Shalamov [4], Yury Dombrovsky [5], Georgy Demidov [6], Yevgenia Ginzburg [7], etc. etc. But the most famous in the West is a mediocre, albeit convenient Alexander Solzhenitsyn that has suited the propaganda needs, hated the Soviet Union and was willing to pull various "facts" out of his ass. As a famous exiled Russian writer and a fierce anti-communist, Sergei Dovlatov [8] once said: "Communists I hate the most, but the ones that I hate even more are the anti-communists".

[0] https://de.wikipedia.org/wiki/Georgi_Alexandrowitsch_Wainer

[1] https://scepsis.net/library/id_128.html

[2] http://flibusta.net/b/201844/read

[3] https://en.wikipedia.org/wiki/Doctors%27_plot

[4] https://en.wikipedia.org/wiki/Varlam_Shalamov

[5] https://en.wikipedia.org/wiki/Yury_Dombrovsky

[6] https://en.wikipedia.org/wiki/Georgy_Demidov

[7] https://en.wikipedia.org/wiki/Yevgenia_Ginzburg

[8] https://en.wikipedia.org/wiki/Sergei_Dovlatov