The lady reviewer found the book arousing. It's about sex, surrender (or sacrifice), and individuality or lack thereof in acts of surrender or sacrifice.
The author of the piece at Quillette says the book is not about sexual fantasies (male and female), but it is.
If one must always search out one’s own desires in order to correct them, then we are suddenly no longer in Aury’s tender world of flesh and fantasy, but rather in Orwell’s world of surveillance and paranoia. Once we moralize the inner landscapes of our own erotic imaginations, even our explicitly pornographic fantasies, what pleasures are left to us that aren’t merely ruthlessly self-serving? And what is left private? It was, after all, a moment of privacy and warmth within myself that I enjoyed while reading O on an airplane. It was a privacy that opened up my erotic imaginings, that deepened me and, ideally, planted the seeds of private fantasies to be shared and discovered with a lover. For sometimes even privacy is best shared with a co-conspirator in laughter, and imagination, and sensuality, and in secret touches and all things forbidden. Things wicked and sweet and brutal and tender. My own private Chateau.