[ 2025 update] I have read much of Ouspensky's other work (Psychology of Man's Possible Evolution, In Search of the Miraculous, The Fourth Way, Tertium Organum) so I can't judge this completely objectively. I have not read Strange Life of Ivan Osokin but I do want to do that someday [now I have ].
The front flyleaf copy on the dust jacket does an excellent job of summarizing the two novellas in this volume, which were originally published in Petrograd in 1916. It was practically a miracle that a copy was found in the British Museum by J. G. Bennett, a disciple of Gurdjieff and Ouspensky (and a brilliant thinker in his own right). Bennett arranged for the work to be translated and published in English, and provides an illuminating and perspicacious introduction. I'm also well acquainted with Gurdjieff's works so that might provide another perspective that not everyone else is going to have on this book.
One can't help but think that Gurdjieff borrowed the concept of these stories in his magnum opus "Beelzebub's Tales to His Grandson." The idea is that what modern man thinks of as "The DEVIL" as equated to "EVIL" has developed and been cultivated over millennia by religious and spiritual persons and movements who disapprove, or are frightened of, any personal philosophy or spiritual path which emphasizes and encourages the autonomy of the individual and free development of their own spirituality over adherence to ritual, tradition, and compliance, and unquestioning obedience.
I personally recommend it, but that's because I'm weird. Your mileage, as they say, may vary.