Inside the flap of Rise and Fall of the Third Reich, by William Shirer.
If you think you know what kind of person Adolph Hitler was or how the Nazi Party came to power but have not read this book, think again. There are things in this book that enlighten everyone as to what actually happened and how Hitler actually became the dictator of Germany. Unlike what some think, Hitler did not get a majority vote. It is a very long and complex explanation as to how everything happened, and it wasn't due to some simple reason such as "Nazis were evil."
In those times, you could have been with the Nazi party due to coercian. Spies were placed everywhere. A spy could have been a friend, anyone who delivered to you, your secretary, even members of your family. If the Gestapo learned of your non-Nazi beliefs, you, too, would be taken to the concentration camps. You learn that the name, Gestapo, in fact, is an acronym, coming from two German words: Geheime Staatspolitzei, meaning Secret State Police.
There are countless things you learn about the origins of Hitler, why Germans felt the way they did at the time and how a man whom most people did not like came to power. With World War II having such an impact on us, still, this book is a must-read.