I read the book Sybil while I was still in High School and it was one of the most interesting books I have read. It is a true story about a girl named Shirley Ardell Mason and it follows her through her life, her life is a little more complicated then most. Sybil is a girl that has multiple personality disorder, also known as dissociative identity disorder, she seemed to have associated with around 16 different personalities. She associated with these different personalities throughout her years gaining more and more personalities as she became older. In the book she becomes co-conscious of her personalities as her therapist Wilbur attempts to integrate her multiple "alters" so that they can interact and gain the knowledge of the other selves. In the end she is able to integrate into one whole individual that is able to remember everything from her past and present life.
Here is a link to a short video that I found on youtube, it shows a clip of Sybil the movie.
Now, here is a little more information on Dissociative Identity Disorder. This disorder "is a severe form of dissociation, a mental process, which produces a lack of connection in a person's thoughts, memories, feelings, actions, or sense of identity." (webmd.com) It is believed that this disorder usually stems from a traumatic experience and this multiple personality disorder is used as a coping mechanism. The way that this is used is that they literally are to traumatized by this experience that they do not want to assimilate with their conscious self, so rather they literally disconnect themselves from the situation, and gain an "alter." That is a brief idea of what this disorder actually is.