Pretty Little Liars is a series of young adult novels by Sara Shepard. The series follows the lives of four girls — Spencer, Hanna, Aria, and Emily — whose clique falls apart after the disappearance of their leader, Alison. Three years later, they begin receiving text messages from someone using the name "A" who threatens to expose their secrets — including long-hidden ones they thought only Alison knew.
For the book series go here
and for the show go here