How much real horror can a reader take?

In the very week that A M Homes won the Women's Prize for Fiction, for her wonderful May We Be Forgiven, our reading group happened to be discussing her earlier novel, The End of Alice, narrated, like Nabokov's Lolita, by an incarcerated paedophile murderer. It's an important book, I think, but for our group its power was somewhat negated by the fact that several members found it just too horrifying to read. An account of our discussion is here on my author blog.