“Pretty Little Liars (Pretty Little Liars #1)” by Sara Shepard
Genre: Young Adult Fiction, Mystery
Summary from Goodreads:
Three years ago, Alison disappeared after a slumber party, not to be seen since. Her friends at the elite Pennsylvania school mourned her, but they also breathed secret sighs of relief. Each of them guarded a secret that only Alison had known. Now they have other dirty little secrets, secrets that could sink them in their gossip-hungry world. When each of them begins receiving anonymous emails and text messages, panic sets in. Are they being betrayed by some one in their circle? Worse yet: Is Alison back?