Pretty Little Liars by Sara Shepard

Pretty Little Liars is a series of young-adult novels by American author Sara Shepard, beginning with 2006's inaugural entry of the same name. The series Pretty Little Liars follows the lives of four girls – Spencer Hastings, Hanna Marin, Aria Montgomery, and Emily Fields – whose clique falls apart after the disappearance of their leader, Alison DiLaurentis. Three years later, when the girls are juniors in high school, Alison's body is found, and they begin receiving various messages from someone using the alias "A" who threatens to expose their secrets and get revenge.
The Pretty Little Liars novels explore several serious issues such as bullying, murder, eating disorders, homosexuality, peer-pressure, infidelity, and mental illness. Moral ambiguity and the consequences of lying are featured prominently in the series: The girls constantly create their own problems through their unwillingness to tell the truth about certain events and misdeeds they have done. The novels have frequently appeared on the The New York Times Best Seller list. In 2010, a television series based off the novels began airing. Stunning, the eleventh book in the Pretty Little Liars series, was released on June 5, 2012. A twelfth book, Burned, is scheduled to be released on December 4, 2012.
The series Pretty Little Liars is loosely divided into three arcs of four books each, chronicling the introduction and reveal of each "A". The series follows the lives of four teenage girls — Aria Montgomery, Emily Fields, Hanna Marin, and Spencer Hastings — whose clique falls apart after the disappearance of their leader, Alison DiLaurentis... (Summary by Wikipedia)