User blog comment:The Koromo/Horror Novels that Shook You to the Core/@comment-25340844-20140927172407

The Girl Next Door, by Jack Ketchum. It's about a young girl who's sent to live with an aunt. The aunt tortures her, and encourages her own kids and those in the neighbourhood to join in, which they often do. The most fucked up part is that it's based on actual events. A lot of the things described in the book happened to a girl called Sylvia Likens, who eventually died as a result of the abuse she suffered.

Wikipedia article for anyone who is interested.