Of the hundreds of sources used for my Ph.D. and subsequent research, I have selected only those secondary sources which are either major, classical works on the topic (and so, although often older, still readily available) or those books which stood out from the many thousands on the topic for one reason or another.

Historical Sources

• Hoffmann, Peter, History of the German Resistance, 1933-1945 This is a standard and comprehensive work on the Resistance for serious scholars.
• Hoffmann, Peter, Widerstand, Staatsstreich, Attentat. Der Kampf der Opposition gegen Hitler
• Schlabrendorff, Fabian v., Offiziere gegen Hitler Released in 1946 this was the first book ever published about the German Resistance; Schlabrendorff was a minor participant, who had also been arrested and tortured by the Gestapo.
• Zeller, Eberhard: Geist der Freiheit Released in 1954 by a personal friend of many members of the Resistance it conveys the spirit of their struggle far better than many later, academic works.
• Fest, Joachi, Staatsstreich. Der lange Weg zum 20. Juli
• Klemperer, Klemens von, Enrico Syring & Reiner Zimmermann, Für Deutschland. Die Männer des 20. Julis
• Lill, Rudolf & Heinrich Oberreuter (Hrsg), 20. Juli. Portraits des Widerstands
• Goddard, Donald, The Last Days of Dietrich Bonhoeffer
• Bonhoeffer, Dietrich, Letters and Papers from Prison
• Hansen, Richard, A Noble Treason: The Revolt of the Munich Students Against Hitler A good introduction to the White Rose resistance group.
• Grunberger, Richard, The 12-Year Reich: A History of Nazi Germany, 1933-1945 By far one of the best, concise social histories of Nazi Germany providing insights into education, the role of women, business, agriculture, the press etc.; rather than focusing on political events, it describes how society worked in Nazi Germany.
• Grube, Frank & Gerhard Richter, Alltag im Dritten Reich. So lebten die deutschen 1933-1945
• Wheeler-Bennet, John, The Nemesis of Power: The German Army in Politics, 1918-1945
• Wheeler-Bennet, John, Die Nemesis der Macht. Die deutsche Armee in der Politik, 1918-1945
• Goerlitz, Walter, History of the German General Staff, 1657-1945
• Dupuy, T.N., A Genius for War: The German Army and General Staff, 1807-1945
• Müller, Klaus Jürgen, Das Heer und Hitler
• Siedler, Franz: Blitzmädchen, Die Geschichte der Helferinnen der deutschen Wehrmacht im Zweiten Weltkrieg

Personal Accounts/Autobiographies

• Wartenburg, Marion Gräfin (Countess) York, Die Stärke der Stille Gräfin Yorck is the widow of a founding member of the Kreisauer Kreis, Peter Graf Yorck and gives important insights to this civilian resistance group.
• Feuersenger, Marianne,  Mein Kriegstagebuch. Führerhauptquartier und Berliner Wirklichkeit Ms. Feuersenger was a secretary working in key Wehrmacht offices including at Hitler's HQ but with close ties to members of the Resistance.
• Boveri, Margaret, Tage des Überlebens Life in Berlin during the war.
• Bredow, Ilse Gräfin v., Deine Keile kriegste doch
• Maschmann, Melita, Fazit Written by a young woman who became a senior leader in the Bund deutscher Mädel - the girls equivalent of Hitler Youth - this is one of the most critical and honest accounts of life inside Nazi Germany I have ever run across.

Novels

• Solmssen, Arthur, A Princess in Berlin Written by a Berliner, son of a Jewish banking house, this is one of the most insightful books ever written about Germany in the Weimar Republic.
• Schrader, Helena P., An Obsolete Honor Novel of historical fiction about the German resistance to Nazism in WWII and of the Valkyrie Plot to assassinate Adolph Hitler.