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.
Nothing specific, but you might wanna check Kristen Ghodsee's work. She can be kinda lib but, at the very least, her sources might be useful to you.