Finally managed to find this book. The author's voice certainly sounds like Nietzsche's, and there are many intelligent and interesting passages that resonate with Nietzsche's "officially verified" philosophical concerns. If it is a forgery, the forger must be some kind of a genius in their own right, displaying detailed knowledge about Nietzsche's biography, and also familiarity with many subjects Nietzsche himself was familiar - and occupied - with: Greek mythology, philosophy, literature, history, theology... So, bravo for the (supposed) forger. Also, even though it could be said that a certain theme of despair and doubt about Nietzsche's philosophy "being built on sand" runs through the book, and it could be interpreted as a Christian maneuver to point out how Nietzsche, "The Antichrist" himself, was finally forced to resign from his Promethean hubris, this is done very subtly; there's no aspect of bad caricature that would stick out immediately. What I mean is that whatever the possible forger's motive may have been (if not just money) in writing this book, they clearly engaged with Nietzsche properly and tried their best to "embody" him.
I saw it said that there was a hint of pornography to the book, which I don't fully agree with. Of course some parts - and there is a certain repetitiveness in the book when it comes to these topics, which I took as a device to underscore the obsessive nature of the madhouse musings of a philopher - are sexual in nature, but I thought they tried to convey something more psychological or philosophical about the nature of intimacy, sexuality, and so on. The incest as a theme raises some questions. Why should anyone come up with incest specifically? I don't know. If this book was supposed to be read as an authentic Nietzsche biography, I would believe it; it would fit well to what is known about Nietzsche, although, of course, nothing in Nietzsche necessitates it.
'My Sister And I' is a very interesting book in its own right; I would even say *good*. Too much about this book's reception has been boiled down to the questions of authenticity and authorship. I'm surprised that those "death of the author" theorists aren't more interested in this book, since wouldn't they have a field day with it? However, some acquaintance with Nietzsche's official work may be recommended to understand the philosophical themes in this not-so-official work more properly.
I would have bought it as an authentic Nietzsche book, if not for a one little detail: at one point the writer mentions his 'Will to Power' (a book) and certain contents of that book. Game over. 'My Sister And I' was obviously written before it had been established by detailed philological research that the book we know as Nietzsche's 'Will to Power' was a composition never intended by Nietzsche as such, compiled from his notes by his sister and his publisher, and published only after his death.
I should also note that, in my eyes, this book doesn't "discredit" Nietzsche's philosophy in any way. It's almost in Nietzsche's own vein, a perpective, a lie --- "Why not lie?"
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There were also some biting and funny remarks in the book. Here's one about Schopenhauer:
"...who wrapped himself in the Buddhist veil of Maya, while he pulled the corsets off women with his brutish hands and threw an elderly seamstress down a flight of stairs to demonstrate his gospel of love and pity."