Although I found it took a while to get going (largely I suspect because the doings of teenage girls don't hold my attention these days!) I found the concept amazingly good. Alex, a nineteen year old girl, begins to see a demon who is visible to no-one else. The fact that he is devastatingly attractive helps him to persuade her to follow his suggestions. Later, as she tries to back out, he uses more forceful tactics. He informs her that humanity is being taken over by an alien species and she can tell by spotting the blue smoke coming from their eyes. He persuades her that for the good of humanity and for the sake of her own life, these people must be killed. This is a huge change of viewpoint on life for Alex. Is she delusional? Is he a figment of her mind or is is all true? She can, after all, see the blue smoke.
Once the demon began to exert his influence over Alex I found the tension and excitement mounted quickly for me. There are some 'hot stuff' moments when the demon uses his sexual influnce over her and some horrific scenes when Alex tries to resist. On one occasion he takes her out to prove it's all true and the results are gruesome. I was by then totally on her side and wanting a good outcome.
Eventually it's information from her past which comes to her aid. I didn't quite expect (but really enjoyed) the ending. There are still questions hanging after the book finishes but the sign of a good story is that you want more! A great read.