Although Hitler was a human, indeed he was a monster. It is human to unfairly dislike something sometime in your life. I am sure most of us may have at one point in our past had a muttering of dislike for a particular race/religion/etc. However, what makes a monster is taking that dislike, then acting on it and killing over a million Jews and other undesirables in the clinical and orchestrated manner he and his cronies did it in.
AS for the German people, yes they need to shoulder some of the blame for supporting him at the time, but weigh in the fact that they were at the time suffering the effects of the Allies excessive sanctions and punishments of World War 1, and the failure of the Wiemar Republic. A form of Stockholm Syndrome occurred, and they saw Hitler as a savior. It is the mark of most "popular" monsters to be able to convince their citizens to follow their madcap means. Also remember many Germans did see through Hitler and tried to fight back, and most paid with their lives (see the white rose group).
Going back to the topic, I agree that this book should be published, so that people can see how a monster is formed, and with the disturbances currently going on in Europe and the world, maybe its a warning to prevent the same thing that happened in the past happening again. We do not know* of any monsters on the scale of Hilter since he died, but that doesn't mean we will never see one again, and maybe the next one will be worse.
*Note i said "know of" rather than outright say that such a monster does not exist. The reason is, even in the case of Hitler, the world in general only knew how much of a monster he was AFTER he was defeated.