The Flight of the Alone to the Alone, by Osho.
It is about the Kaivalya Upanishad:
https://en.m.wikipedia.org/wiki/Kaivalya_Upanishad
https://www.google.com/search?q=flight+of+the+alone+to+the+a...
https://www.sannyas.wiki/index.php?title=Flight_of_the_Alone...
I have read that book. It is good.