You by Caroline Kepnes

When aspiring writer and recent Brown graduate Guinevere Beck strides into the bookstore where Joe works, he's instantly smitten. Beck is everything Joe has ever wanted: she's gorgeous, tough, razor-smart, and sexy beyond his wildest dreams. Joe needs to have her, and he'll stop at nothing to do so. As he begins to insinuate himself into her life - her friendships, her email, her phone - she can't resist her feelings for a guy who seems custom-made for her. So when her boyfriend, Benji, mysteriously disappears, Beck and Joe fall into a tumultuous affair. But there's more to Beck than her oh-so-perfect façade, and their mutual obsession quickly spirals into a whirlwind of deadly consequences.