The Girl on the Train by Paula Hawkins

Through the train window each morning, Rachel sees a couple she has named "Jason and Jess." She imagines that they have the perfect relationship, but they don't. Megan (Jess) is floundering career-wise and is having an affair. When Megan goes missing, Rachel insinuates her way into Scott's (Jason's) life and the police investigation. Yet Rachel has problems of her own. She is an unemployed alcoholic who drinks so much she can't remember where's she's been or what's she done. One of her blackouts happened on the night that Megan disappeared. It turns out that Megan and Scott live down the street from Rachel's ex-husband, Tom and his new wife, Anna. Rachel just can't leave Scott and Anna alone, especially since it was Anna that was partly responsible for the breakup of her marriage. Unreliable, dislikable characters and a possible violent crime highlight this suspenseful tale--a good read-alike for Gone Girl.