The Girl on the Train: A Novel - Paula Hawkins
The story goes back and forth between three women.

Rachel takes the same train into town every weekday. The train stops at the same signal allowing Rachel to look out at the couple who are usually on their deck. She has named them and she knows everything about them. Their lives are perfect. But one day she sees something and what she has imagined for this couple is shattered. Wanting to help, she tells the police what she saw, but being a drunk no one really takes her seriously.

A definite suspenseful page-turner. There was one part towards the end where I didn't like the book after that, otherwise, obviously, it would have been a five star book.