However, it is quite well documented that the German re-armament, forbidden by the Treaty of Versailles, was largely made possible by this co-operation with USSR. It provided grounds for training armored forces in Soviet Union, allowed building and testing chemical weapons in Soviet test sites under German leadership, and had German aircraft tested in the USSR. The sources I read before this were not in English but after a quick Google one could start with e.g. http://www.feldgrau.com/ger-sov.html
It looks like "The Roots for Blitzkrieg" is an English presentation that I should read myself (thanks to prompting the search).
The Germans visiting Soviet extermination camps is documented at least in "Kremlin kellot" ("The bells of the Kremlin") by Arvo Poika Tuominen (a Finnish Communist who worked for Komintern in 1933-1939 and spent quite some time observing the camps at the Baltic-White Sea Canal works and Solovki prison.) It's a pretty chilling read; the canal works were where the basic research for extermination via forced labour was done.
Solzhenitsyn then later provided a more well-known description of the GULAG.
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