Everything you need to understand or teach You by Caroline Kepnes.
In the novel You, written by Caroline Kepnes, bookstore manager Joseph “Joe” Goldberg becomes obsessed with Guinevere Beck after she visits his bookstore. Using social media, as well as a phone he steals from Beck, Joe stalks Beck. Joe's constant stalking presents an opportunity for him to save her after she falls onto the subway tracks. Joe, who shows signs of being a psychopath, is so obsessed with Beck that he is even willing to murder to ensure that she is his. Themes in the novel include stalking in the age of social media, the importance of education, and the characteristics of a psychopath.