* The classic mystery novel "The Secret Garden" by Frances Hodgson Burnett. Shepard has said that she loved the idea of a group of young girls with a secret, and that "The Secret Garden" was a major inspiration for the setting and the characters in Pretty Little Liars.
* The teen drama television series "Gossip Girl." While the show wasn't released until after the first Pretty Little Liars book was published, Shepard has stated that the show's focus on wealthy, troubled teenagers in a gossipy environment resonated with her.
* The works of Agatha Christie and other mystery writers. Shepard is a self-proclaimed fan of mystery novels and has said that she draws inspiration from the classic whodunit format, particularly the use of red herrings and unexpected twists.
* Her own experiences as a teenager. Shepard has admitted that she drew on her own memories of high school to create the characters and the dynamics of the group of friends in Pretty Little Liars.
While those are some of the most prominent sources of inspiration, Sara Shepard has also mentioned other influences, such as the works of Stephen King, the film "Scream," and her own personal experiences with bullying and gossip.
Ultimately, the creation of Pretty Little Liars is a combination of these influences, and her own unique talent for weaving suspenseful and engaging stories.