To help you capture the twisted and gangsterish world that was the Nazi government, I'd highly recommend you pick up William Shirer's
The Rise and Fall of the Third Reich. Shirer was an American journalist working in Central Europe (Germany, Austria, Czechoslovakia, etc.) from when the Nazis took power until the early stages of World War II. A couple decades after the war ended, Shirer used his first-hand experiences, his contacts, and the voluminous records the Nazis left behind to craft the 1000+ page opus that is
Rise and Fall.
(Hell, I'd recommend the book to anyone who wants to understand Nazi Germany.)
The book sheds light on the major resistance groups within the intelligensia and the military, whom you might want to incorporate into your plot at some point. Maybe after the hero is conscripted into the Wehrmacht, Admiral Canaris of military intelligence sinks the claws of the anti-Nazi officers into him.
(Normally, we'd think of shadowy intelligence types as being pro-dictator, but Canaris and his deputy, Colonel Oster, sought to topple the Reich.)
For even more insight into the German resisters in the officer corps, you might want to nab
Plotting Hitler's Death by Joachim Fest.
I know you're not writing a history book, but knowing the history well would help you craft more believable settings and characters.
