Do you have any evidence that there is a susceptible set? Books especially fiction come with two fundamental safety features. Firstly the book never subjects itself upon its reader. The reader is always in absolute control. Secondly the fundamental unreality of the setting allows one to dismiss it.

I recall observing with morbid curiosity a video of someone losing their life to a terrorist many years ago thinking it would be no different than a gory movie. I was very very wrong. It isn't the realism of the simulation that matters its the inability to dismiss what one knows is in fact real.