Timothy Snyder's Bloodlands--a NewYork Times bestsller--Hitler, Stalin, Ukraine, Russia, Poland, Jews--

https://www.amazon.com/Bloodlands-Europe-Between-Hitler-Stalin/dp/0465031471?asin=0465031471&revisionId=&format=4&depth=1

I've been to Auschwitz and Majdanek in Poland.

This is a must read--also Robert Conquest,The Great Terror.