This book is disturbing. The book is a collection of evidence - diary entries, transcripts of video clips, interviews with students and a psychologist - surrounding a fire at Elmsbridge school. The only suspect is a girl with Dissociative Identity Disorder (multiple personalities) who also died in the fire. While the book keeps counting down to "The Incident," there are many traumatic incidents that happen before the beginning of the book and during the countdown. The main character's (Carly/Kaitlyn Johnson) parents died before the beginning of the book. The main character is checked into a mental hospital of some sort and then sent to Elmsbridge boarding school. A student goes missing, several students die, the misery is nonstop. This is reminiscent of Shutter Island in that you can't trust the information from the main character as she devolves into psychosis - or demon possession, depending on the interpretation you prefer.
Amazon states this book is YA. I personally wouldn't let a child under the age of sixteen read the version I listened to on audiobook. There is apparently an alternate paperback edition for grades 8 and up. Parents will want to preview this before giving to a child - language, disturbing themes, occult/magic/demon possession, sexual situations, violence.
There is a companion novella The Dead House: Naida.