Appearances are all too often deceptive. I wrote this about one of the previous books I read, but in "The Couple Next Door" this theme seems to be even more prevalent.
A young couple visits their next door neighbors for a late dinner. They decide to leave their infant daughter at home and they take a baby monitor with them. As the dinner prolongs, they take shifts in checking on the baby. Nothing unusual happens until they return home and realize that a serious crime has been committed.
As new facts are revealed in the investigations, surprising twists in the plot make the action more dramatic. New facts about the protagonists are revealed, and their motivations seem to start making sense only to lose it again after new details are revealed. It is a very well designed narrative. The moment everything begins to start making sense, a new dynamic is added that adds a new level of complexity.
I found it hard to put it away as I was sucked into the story. It is very well written, often surprises the reader, and has one of the best final lines I've even seen in a novel.