The Girl on the Train by Paula Hawkins,

Rachel catches the same commuter train every morning. She knows it will wait at the same signal each time, overlooking a row of back gardens. She’s even started to feel like she knows the people who live in one of the houses. “Jess and Jason,” she calls them. Their life—as she sees it—is perfect. If only Rachel could be that happy. And then she sees something shocking. It’s only a minute until the train moves on, but it’s enough. Now everything’s changed. Now Rachel has a chance to become a part of the lives she’s only watched from afar. Now they’ll see; she’s much more than just the girl on the train...