This book is a satisfactory summary of the philosophy of Friedrich Nietzsche. Having read this, I feel like I understand Nietzsche's philosophy, and I have no lingering curiosity towards reading his own words in order to understand it any better (primarily because the philosophy of Friedrich Nietzsche is.... not very good, from what I can tell).
It is also, I think, a summary of the philosophy of HL Mencken, who is an enthusiastic fan of the man who provides his subject matter. Normally I think I would rather hear an intelligent critic's summary of any given philosophy than a rabid fan's, but in this case Mencken's outlook somewhat adds to the reading experience.
There are probably not a lot of people these days, other than maybe the type of person who listens to death metal, who can subscribe to Nietzsche's philosophy entirely. This is because on the one hand, Nietzsche is a fan of science, societal progress, and rejection of traditional morality, typically associated with the left. But on the other hand, he is an advocate of aristocracy and the subjugation of the weak to the strong, typically associated with the right. However, Mencken's own views combine these two outlooks. In fact, Mencken is known for his mockery of American white rural Christianity from an elitist aristocratic perspective. This is a practice which in our age has been taken up by those who are ostensibly on the left and in opposition to hierarchy, but in Mencken it is associated with an open contempt for democracy.
As the book develops, it's hard not to perceive a shift away from Nietzsche's actual views into an increasingly frenzied defense of many social Darwinist doctrines that most today would consider somewhat reprehensible. I knew that Mencken was a racist, but I was under the impression that this was a fact only revealed in his personal diary entries, published posthumously. In fact, midway through this book, he casually comes out in favor of African-American slavery. The chapter "On Women" is also... interesting, in the sense of "wow, this is what you could write in the year 1907". This transition into an elitist polemic is somewhat entertaining to read and provides a good complement to the exposition of Nietzsche's philosophy, perhaps shedding light on the sort of mindset that his writing might lead to.