Books designed to shock. The message of the story (if there is one) is delivered through writing that's meant to push readers to the edges of comfort.
From Rene Chun at The New York Times:
A literary genre that graphically explores such topics as incest and other aberrant sexual practices, mutilation, the sprouting of sexual organs in various places on the human body, urban violence and violence against women, drug use, and highly dysfunctional family relationships, and that is based on the premise that knowledge is to be found at the edge of experience and that the body is the site for gaining knowledge.
Many of these books come from godless.com.
