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 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 novels explore several serious issues such as bullying, murder, drug addiction, underage drinking, 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 Pretty Little Liars (TV series) loosely based on the novels began airing. The twelfth book in the series, Burned, is scheduled to be released on December 4, 2012. A prequel, Ali's Pretty Little Lies, is set to be released on January 2, 2013, is set to be released in June 2013.