I'm not advocating a rating system.  But all forms of popular media so far have had a parental advisory statement of some kind- music, movies, television, video games...

Novels have remained relatively untouched.  One can pick up something from an author and find dirty words, violence, and sex without anyone complaining, then pick up another book from the same author and find none of these descriptions.  Really, the only thing seperating mature content is the "young adult" bookshelf and "every other bookshelf" in the store or library.  But kids aren't prevented from reading an "adult" book, and in many cases these books aren't any more adult in content or even more mature.  They may be less unintelligent compared to young adult books, but not always.

So why have novels been unscathed from concerned parents that their favorite science fiction or fantasy novel contained descriptions of sex or the word "fuck"?  Is it the reliance on imagination that seperates words from movies and TV?  Is it the extra effort and will that makes reading more voluntary than an involuntary activity such as watching a film?  Or do people just read what they want, preventing social and religious activists from even coming across certain books unless unless by sheer accident?

Note there have been plenty of banned books, mostly when those books have been introduced in elementary school libraries.  But I'm asking why there is no rating system such as "mature" or "contains descriptions of" statements on the cover.  Even comics say "For mature readers" or "for adults only," but there is little to differentiate one novel's content from another.  I'm not advocating or suggesting this be done in any way, I'm just asking why it hasn't happened yet with our multitude of concerned parents.