The Girl on the Train by Paula Hawkins
In her debut novel, The Girl on the Train, Paula Hawkins tells the story of Rachel Watson, a recently fired alcoholic whose life spirals out of control after her ex-husband marries his mistress, Anna, and the two have a baby. To pass the time, she rides the train into London each day, whizzing by the house that she used to live in, as well as the home of a seemingly happy couple whom she calls Jess and Jason. Day after day, she imagines what their perfect marriage is like and it helps her cope with her own struggles. However, her fantasies come crashing into reality when Megan Hipwell, aka Jess, goes missing. Rachel is determined to get down to the bottom of what happened . . . but she can no longer separate the truth from the lies she's created.