There is Corum's book about Von Seeckt's army reforms. The Roots of Blitzkrieg.
Lots of information is available online concerning the German-Soviet R&D joint efforts, which is something that can be pretty interesting. You should look up the tank school of Kama and the flight school of Lipetsk.
Infiltrating either, for a Western agent, would be really very hard.
Here's an example, a bundle of original documents from the BAMA (but it's in German).
https://www.bundesarchiv.de/oeffentl...ndex-1.html.de