Ian Kershaw & Richard Evans have written extensively on Nazi Germany so any of their books would be a good shout. Good introductions by them and others would include something like:

• The Third Reich Trilogy by Evans (The Coming of the Third Reich, The Third Reich in Power & The Third Reich at War)

• Hitler, 1889-1936: Hubris & Hitler, 1936-1945: Nemisis by Kershaw (which was abridged to just Hitler)

• The "Hitler Myth": Image and Reality in the Third Reich by Kershaw

• Weimar Germany: Promise and Tragedy by Eric D. Weitz

• The Third Reich in History and Memory by Evans

• A Concise History of Germany & Germany, 1918-2014: The Divided Nation both by Mary Fulbrook might be good general overviews of Germany's recent history.

• The Rise and Fall of the Third Reich & The Rise and Fall of Adolf Hitler both by William L. Shirer might also be a good shout even though they're old they were written closer to 1945 than any of the other books mentioned so that might work in your favour.

There are innumerable books on this period, all of which have some strengths. To find more books about this period you can view the subreddit book list hopefully this helped :)

You might be able to find some of Robert Gellately's work on the Gestapo online.