Most of the historic record of the GDR has been set by FRG historians running victory laps and is based on anecdotal evidence at best, mythology at worst. A lot of people also attempt to equate the GDR with the third Reich (which is insane to begin with, but that’s what people are taught here).
Preferred languages would be German and English. I could make Russian work, but not very well.
Edit: I know about John Green.
Victor Grossman is a journalist who has mostly published works in German.
I’m not certain as to how scientific it is, as in like hard data, but if you’re looking for that sort of stuff then my guess is that his work wouldn’t really hit the mark.