I don't know much about ww2. I've read a few books about it but the nitty-gritty of the war doesn't interest me. However, ever since I read my first book on the holocaust I wanted to read more about the transition.
(What follows are my own uneducated words):
Then, most importantly, how can a society like that change (what I consider) so quickly into feeling guilty about what they did.
In historical fiction I remember this Jewish woman after the war saying how no matter how far away from Germany she went, she still felt under threat because all Europeans were somewhat ok with what had happened (totally my words but just the vibe)
Hanna Arendt's "Eichmann in Jerusalem: A Report on the Banality of Evil".
Hanna Arendt was a political thinker who was sent to Israel to cover Adolf Eichmann's trial for the New Yorker. In her book she covers many subject but the most memorable one is her attempt to explain how a ordinary/nomative person can become actors in a totalitarian system.
Ordinary Men : Reserve Police Battalion 101 and the Final Solution in Poland by Christopher R. Browning
I'm warning you, this book is gruesome. The police battalion 101 was composed of middle aged men, mostly city policemen. In 1941 and 1942 they conducted what was later called "the holocaust by bullets", the rounding up and mass shooting of Jews on Poland and Ukraine. In the book Browning tried to explain how these ordinary people became able to conduct such atrocities. He especially talks about how social pressure and group dynamics pushed them to conform and carry out orders.