The source material for the iconic film of the same name, The Exorcist is a about a young girl and what resides within her.
You follow Chris and her 12 year old daughter, Regan, as Regan's behavior begins to change and morph until she becomes unrecognizable to those around her. As her condition worsens, new characters fall under your eye: Klein, Father Karras, Kinderman, and more, all with their own involvement with Regan, Chris, and the events around them.
I enjoyed the book, though I thought the begin was a bit slow, personally. I enjoyed the book, but it wasn't a four star book for me. I would give it a read, especially if you have seen the movie (I haven't seen it, but I digress). It is a bid crude in a couple of places, so if disturbing descriptions make you uncomfortable or something similar, prepare or shy away from this one.
Horror readers and newcomers, enjoy!