I actually was considering this for four stars, but decided to keep it at three and I will attempt to explain why.
When the story begins, I had a difficult time getting into the story. I knew something was off about Nick and his reactions to things, but Alan was equally off and I spent over a hundred pages trying to place what was wrong. And Mae always felt weak as a character to me. Her "character" was strong and devoted to her brother and determined to save him and etc. But that was the problem. She was just a bit character for me and never felt like a real personality. And that made a lot of scenes fall very flat that featured her. I honestly could wish she had not been in there.
But as the story progresses and the secrets and lies start piling up, as Nick grows increasingly unstable, to the point of violence toward Alan, the one and only person he cares about, everything coalesced to the moment when I realized, with the acute awareness that all the little moments pointed to it so clearly, that Nick is not human. (I figured it out when Alan said that Nick had been so sick when he had crossed water when he was little.) So I knew for quite a while before the big official reveal, but knowing that Nick was not yet aware still kept me intrigued. But Sarah really did a very nice job I felt of planting the clues and hints and letting them seep in and grow. Obviously, Nick's lack of emotion, his total inability to comprehend the meaning or necessity of physical contact such as hugs and such, his difficulty with words, his ability to spot lies, as well as how utterly callous he seems can, by themselves, just be some wacky mental quirks or deficiencies, but add them all together with Olivia's refusal to let Nick near her and her rabid dislike of him, Alan's comments (about how he believes demons can love and a lot of other remarks), Nick's talent at dancing and some inconsistencies with the whole "Arthur want's Olivia's charm" story and you have a lot of groundwork laid for that reveal, but she did it gradually enough that I spent a lot of time totally unaware and truly believing Nick was human, albeit a very disturbing one. And that was impressive.
There were some difficulties with some of the world-building, such as if humans are possessed, outside of a circle of containment, such as those that are brought to Merris', why don't some of those demons just escape and wreak the havoc that Nick supposedly would have? Some things simply were not explained to my satisfaction.
I liked that she managed to show that although Nick does "feel", he chooses, over and over again, to care in his actions and decisions and his thoughts. He may not feel love and concern, pride and hurt, fear, etc., in the same way as humans, but he does feel them in a different way and more than he realizes from how he is written. He knows these things more as ideas than emotions and he reacts to them differently. I would say he actually has more difficulty recognizing and expressing what he thinks and feels than anything else. (And this sounds like gibberish no doubt as I ought to write this when I am more lucid.)
But the mystery of Marie and in fact Alan's entire backstory was handled so well! And being able to make me feel sorry for Olivia in the end and actually sympathize with her? Sort of unbelievable as I spent most of the book really despising her and totally unable to understand Alan's devotion to her. Then I saw where she was coming from and I had a glimmer of understanding her motivations. Also saw what Alan's actual motivations were as well and who he had been truly concerned for. Best scenes were the boat crossing with Alan and Nick and the very end. And as content as I was with the ending, I am quite interested in where Sarah Rees Brennan will take Nick and Alan in her sequel due out in 2010.