A Head Full of Ghosts is not what I'd normally expect to see in a book where a character is said to be possessed. I mean that in a good way.
This book is told from the point of view of a woman who as a young girl who saw her older sister suffer from schizophrenia. Unfortunately, there father has recently lost his job and found religion. This combination causes him to become convinced that the elder girl is possessed and to get the family to take part in a reality show culminating in his elder daughter's exorcism.
There are no obvious supernatural elements. Instead of being a horror story about a girl's head doing 360° turns while projectile vomiting pea soup, it's about what happens when the horrors of mental illness, religious fanaticism, and the reality TV culture intersect. This makes for the sort of book that is as likely to appeal to people who don't read horror as it will to those of us who do.