The "Pretty Little Liars" book series, by Sara Shepard, follows the lives of four teenage girls – Aria Montgomery, Emily Fields, Hanna Marin, and Spencer Hastings – after the disappearance of their queen bee friend, Alison DiLaurentis. Years later, they begin receiving threatening messages from someone calling themselves "A", who knows their deepest secrets. The series explores themes of friendship, betrayal, and the consequences of keeping secrets, while also delving into darker topics like bullying, underage drinking, and infidelity.