TL;DR: Very good book but its apparently only really relatable for about half the people who read it. Consider it a story of the type of person you don't want to be.
I am writing this review months after I read the book because that's how profound an effect the book has had on me. For that reason, I apologize beforehand if there are any inaccuracies in this review.
To me no longer human is the story of the man who was unable to overcome his cynicism. It is a cautionary tale about the dangers of living in a state of constant paranoia. When I was younger and to this day looking at all the hypocrisies of myself and the people around me I became a more withdrawn person and as a result an introvert.
This book is interesting because, for the protagonist, the opposite occurred. He experienced the same paranoia but in response, he became a clown. His to me is a self-indulgent perspective in which he believes that he alone is different, and for that reason, he is not a real human. However, I think he is mistaken, to me, he is just as human as most other people, if not more so.
As much as he is terrified by the hypocrisies that exist within Japanese society, he is a part of the system he feels oppressed by. To me, he is the biggest hypocrite. He pretends to be a cheerful, entertaining person without ever describing what he truly feels to other people.
Sure he doesn't speak behind other people's backs however instead of actually ever attempting to fix the problem he sees by truly telling other people what he thinks he instead chooses to do nothing at all, even when he sees his wife being raped. In this aspect, I think I could relate (not even close to the same degree, I wouldn't do nothing as someone I love is being harmed, but I could still relate), and this is why I think the story had a big impact on me.
For a long time, I've been trying to change that aspect of myself and I think no longer human has truly helped my growth. Unfortunately, the author Dazai was never able to recover from his traumatic experiences and eventually succeed in killing himself after many attempts (this book can be thought of as almost an autobiography).
To me no longer human is a book that shows me the dangers of the paranoia of hypocrisies. I still see these hypocrisies but at the same time, it's different because I think it made me realize that there are at least a few other people who are out there who are just like me. I'm not alone in that regard. The book has shown me that if I want to move forward I need to try to actually look beyond the hypocrisies and try to understand everyone else around me instead of demonizing them.
That was long, personal, and probably slightly incoherent at points but I hope that this review can give you an idea about what this book can mean to some people. Also yes, I was led to read this by Bungou Stray Dogs.