Never Let Me Go by Kazuo Ishiguro is one of the most haunting books I've read yet. It's a bit slow at first but towards the end of it you feel like a flood's washed over you, and once you closed the book you have to sit down for a while and think through the story again and feel its tragic poignance, at the same time not wanting to believe in it. No matter how hard you try to stay detached, you'll feel something in the end. It's much like a relationship, really.