I think the difference between adds on t.v/magazines that promote an unrealistic body image, is that those adds, which employ various image manipulations, are used as "real" images. People see them and think, dang, if she can look like that, I guess I can look like that.
With books, particularly FICTION, people know going in that they're reading/consuming fiction. Can books impact your life? Sure. It's a form of art and art can influence. Do I think movies and books and sit-coms hurt people .... ever??? NO. A ten year old who reads twilight and thinks, golly gee I want a boy friend like that, will one day grow up and learn what novels are, she'll see countless other relationships and realize that perhaps edward/bella wasn't meant to portray an ideal relationship, but rather it was simply meant to portray a relationship between two fictitious people, only in the context of the story which one author was trying to tell (and did tell, very well, to the enjoyment of some 100 million people or so).