What's the grossest good book you've read?

"But good books disturb for good reasons. To disturb is, among other things, to guard against complacency: to make the reader face the underbelly of dark thoughts and actions, see how circumstances can make even good people go astray if they are not vigilant in honoring the best in themselves and in the outside world. Disturbing passages, when skillful, make a vital inquiry into the subtle causes and effects of human behavior. "

A novelist considers the value of disturbing books.