The group of people who most likely fit what you're looking for are the members of the military resistance to Hitler, who attempted on several occasions to assassinate him, most famously on the 20th of July 1944.

If you want a detailed overview then I'd recommend Peter Hoffmann's The History of the German Resistance 1933-45, which is one of the most comprehensive works out there. However, at 872 pages it's absolutely massive so if you're looking for a thinner work then I'd recommend either Hans Mommsen's Alternatives to Hitler: German Resistance under the Third Reich, Joachim Fest's Plotting Hitler's Death: The German Resistance to Hitler or Ian Kershaw's The Luck of the Devil: The Story of Operation Valkyrie.

There are (relatively) few good biographies of individual plotters unless you speak German, but take a look at Michael Mueller's Canaris: The Life and Death of Hitler's Spymaster or Peter Hoffmann's Stauffenberg: A Family History 1905-44 for starters.