I tend to think that people say these sorts of things so that they can be sure they're not sinning. Sure, on a daily basis I get needlessly angry, treat my neighbors poorly, indulge fear more than faith, etc., but at least I'm not reading a book about vampires or kid witches. It's not always the case, but I really do smell Phariseeism behind a lot of Christian critiques of popular culture.
Anyway, is it a sin? Perhaps, perhaps not. If it is a sin, what percentage can you read and still not sin? The first chapter? Surely the first sentence is ok. What happens if you don't know what the book is about and someone gives it to you to read? Will accidentally casting an eye on the page **** you?
"It is not what enters into the mouth that defiles the man, but what proceeds out of the mouth, this defiles the man."