It's really difficult to talk about this book without being spoilery, so forgive me if it seems like I'm dancing around things and being cryptic.
Things I loved about this book: Almost everything honestly. Specifically though, the voice. Ren is a compelling unreliable narrator. I really appreciated what an excellent job Newman did with conveying what it's like to live with an anxiety disorder. Also, while Ren clearly has a Secret, I wasn't frustrated by the allusions to a bigger untold story like I often am in narratives like this. Ren's POV was so convincing I felt comfortable that the story continued to just hint at whatever happened in the past, because I felt like Ren herself was avoiding thinking about it. I was continually compelled and convinced by Ren's internal struggles.
This is a masterfully written book, and it is also a terribly personal book. This isn't a large sweeping drama. There's very little action. The bulk of the story takes place in a matter of days. It skips through time. It's a psychodrama. And in the end it's all about one woman. This was all a big plus for me because of the execution, but it could be a turn-off for people wanting something less meditative and more action packed.
Also: the science. I loved that this was a hard sci-fi book, filled with interesting speculative science and alien worlds, and it wasn't weighed down in jargon and complex explainers/exposition. The science is simply woven into the story, and makes perfect sense as you read. It isn't the focus of the story, but it is an important part. I didn't realize just how much I was craving a true hard sci-fi story that has such a personal focus until I was reading this book.
Be warned, there is something about this book you may really hate, and that's the ending. When I finished the book I was a bit taken aback, but after sitting with it for a couple days I actually rather like the ending. It feels right to me. However, I am in the extreme minority. Most people (both in my book club and in reading reviews) really strongly dislike the end. So you are warned. I think it is a matter of taste.
If you're looking for a deeply personal, meditative, character study: try this book. If you want to read a hard sci-fi that reads more like a psychodrama: try this book. If you love unreliable narrators, or really enjoyed Annihilation by Jeff Vandermeer: try this book. If you want a sweeping space opera, an action packed sci-fi filled with a kick-ass heroine, or a book with a more traditional structure and ending: skip this book.