Merry was 8 years old when her older sister Marjorie started seeing things and acting weird and causing trouble in the family. They quickly come to the conclusion that Marjorie is possessed by a demon and the whole family goes into turmoil as they determine how to deal with her Head Full of Ghosts. First they agree to become a reality tv show and fill the house with cameras that record all the unusual incidents. Then they send Marjorie to a psychiatrist for counseling and medication, which doesn't work. Ultimately, they bring in a priest to perform an exorcism.
The book is ok, but could be better. The story is told by Merry as she is recounting it to a writer who wants to publish a book about these events. And there are some strange "interruptions" in the story by a blogger, which was out of place and annoying. It was kind of a combination of The Exorcist, Poltergeist and The Omen.... only not scary. However, if you liked those books, you'd probably like this too.