Plotting Hitlers Death - The German Resistance to Hitler 1933-1945 by Joachim Fest.
An incredibly well researched and well written book. In fact the title is misleading in that it covers only resistance to Hitler in the German Military.
It is fascinating in that it the covers no just Operation Valkyrie, the most well know assassination attempt against Hitler, but also other attempts that failed. It considers the actions and motives of the key people involved in the military resistance, both were complex. It gives a clear guide to their world view and the many reasons why the attempts were often cancelled or unsuccessful.
In the end the motivation became
It is a book with unexpected heroism - two plotters telling the Gestapo nothing for 6 days despite unimaginable torture. Other plotters gave themselves up either naively thinking they would be put on trial or just showing by surrendering that they disagreed with the Nazis, something the Nazis found almost incomprehensible.