I read this book with my book club. I was intrigued by the promise of unreliable narrators, because they are a weakness of mine. If you don't care for those, or feel cheated or lied to when you read that sort of thing, you'll want to skip this one. I was also interested by people comparing the author to Gillian Flynn. There are definite similarities. I think if you like one, you'll like the other.
I suppose this book would be considered a psychological thriller, Or a murder mystery, but I'm not very good at identifying the right categories for these things. I do recall, after years of watching shows like Bones and Monk, that you can identify the killer by spotting the unnecessary character. In those TV shows, that always works. In this book, I thought I had done that, but it turned out I was wrong. And I was pleased by the red herring. Again, unreliable narrators make that difficult to find. So, don't expect this to be easy to figure out right away.
The story is very dark. I think it manages to be less dark than Gillian Flynn's books, but not by much. There are three narrators. Rachel is who I would consider to be the main character. She is a divorcee with a drinking problem, prone to drunken blackouts, unable to remember what happens during them. She's also one of the only likable characters in the book. With all of her flaws she still manages to be sympathetic. It can be hard for the reader not to root for her, even while cringing at a lot of her choices.
The book was engaging from the very beginning, with the story drawing you in and keeping you there, because you'll want to know what happens next. I highly recommend it.