I'm glad I have the shelf "horror/suspense," because this book is those things. Psychological horror and somewhat quiet suspense, sure. But horror/suspense indeed. ("Thriller," not so much.) 
 But I digress, if it's still considered digressing to start off on the wrong track. 
 When I got to The Big Twist, what came to my mind was the season of Dexter with Colin Hanks (season 6, maybe?). There was that one episode -- I won't spoil it because it was one of my favorite moments in television and everyone should enjoy it unspoiled, but if you saw that season, you know what I mean -- where the action suddenly bent at 90 degrees and everything got incredibly creepy. This book had the same kind of moment. I actually had trouble believing what was happening when certain things were finally revealed, and I had to read that part twice. But it only got more wild from there, and I so completely love stories like this: when I have no idea where it will end up, but I know that it will be amazing. 
 And if you are like I was before reading the book, and feeling a little scornful of the idea of a "twist," stop it right now. M. Knight Shyamalan wishes he had these twisty skills. 
 I think what impressed me the most about it was that the story was told so perfectly well. It is brimming with perfectly timed information, well-executed setups, and just badass narrative skill. The story goes from a little bizarre to totally crazy to unbelievably fucked up to are you seriously kidding me here fucking insane. And I was right with the characters the whole time, and I found myself falling for their tricks as well. The whole thing was just...okay, yeah, amazing. 
 I'm tempted to say that the relationship between Amy and Nick can be summed up by something Nick said: "It was kind of romantic. Catastrophically romantic." But no, that's wrong. One part of the story is a little romantic, but only in the way that a romance would be between, say, Charles Manson and anyone: romantic, yes, but mostly just horribly dysfunctional and awful. 
 I am starting to feel like I'm giving too much away (although I never would have guessed that twist in a million years, so if you do, congratulations, you're a genius, and I also never want to hang out with you again because you have the mind of a lunatic). But truly, read this book. And feel thankful that these people are works of fiction.