It's hard to say. The novel's description of mental illness could be influenced by the author's creativity rather than strict medical facts. There might be some elements that are fictionalized for the sake of the story.
I'm not entirely sure specifically what 'delivery the beast within based on a true story' is about without more context. It could potentially be a book, movie, or some other form of media that involves a true - story - based exploration of some sort of 'beast within' concept, perhaps related to human nature or a specific event.