This is a comprehensive but perhaps not “definitive” study of the Nazi regime under Hitler. It’s quite readable and although really long, very rarely got boring. It won the National Book Award and was unexpectedly a top seller. Shirer based much of this book on documents which had been released only a bit prior to his starting his research. He also worked as a journalist in Germany between 1934 and 1940.
One issue which crept in from time to time was the idea that Hitler was a natural product of the German mentality. This idea has little or no acceptance today although it was quite popular during WWII and for a couple decades later.