I don't read much mystery or suspense, usually because the writer doesn't take the time to create characters that are more than two dimensional or flesh out the world around them. This author did both, and added  a plot that, as others have pointed out, grows satisfyingly creepier on each page -- so much so that I found myself standing at a cross-walk in broad daylight thinking, "I really need to start paying more attention to who's around me. "  By crossing and recrossing the fine line between ordinary weirdness and quantum weirdness, it kept me guessing well into the late stages. Yes, it's long, but I read the last 130 pages in one sitting on the edge of my seat
This author did both, and added  a plot that, as others have pointed out, grows satisfyingly creepier on each page -- so much so that I found myself standing at a cross-walk in broad daylight thinking, "I really need to start paying more attention to who's around me