[Warning: mild spoilers about 'The Girl with the Dragon Tattoo']
A lot of readers here, so I'm guessing that there are a lot of opinions here on this subject...
My daughter is 11, almost 12. She's an avid reader -- always has three or four books that she's actively reading. She tends to read difficult books (just yesterday she asked me if she should read Hamlet or Rome & Juliet next -- I suggested Hamlet).
As her reading ability increases (it's already beyond 12th grade level) the types of books that she is interested in become more adult in nature, which introduces new challenges for me.
Some time ago my niece texted me to tell me that my daughter was reading the Twilight series [yeah, she still reads popular stuff too] and that she was getting toward the fourth book, which wasn't appropriate for her age. I downloaded it and skimmed the book. I didn't find anything overly objectionable, no worse that you'd see on daytime TV, so I decided not to stop her from reading it (though I considered objecting based on the quality of the writing, but that's a different issue

). Anyhow, I've mostly taken the view that sheltering her isn't the right approach. I grew up in a sheltered home, prevented from seeing the world around me, and feel that my parents did a disservice. I left home unsure of who I was, what I believed, etc... I'd still been exposed to the things that my parents tried to keep me from through classmates, but never had the opportunity to properly think about them, and definitely no way to discuss them with an adult. I often wish that I could go back in time and tell my child-self a number of things about life. For this reason I am happy to see that my daughter has a much more informed view of life than I did and that she's willing to talk to me about the content of what she reads.
That said, there's still a line (I think). Problem is, where is it? Yesterday, somehow, the book 'The Girl With the Dragon Tattoo" came up, not sure how. I said that it wasn't really a kid's book, and she said something like "Oh, I'm glad that you told me, I was about to read it." I didn't elaborate, only said "Yeah, it's probably better that you wait for that one."
I'm not sure why I felt that way... it's not significantly worse than other things that she's reading. What makes it different? The sadism? Sadistic things happen is this world, and they're not condoned in the book. Do I object to the casualness with which multiple sexual relationships are discussed? I'm fairly liberal, so I don't think so -- I don't find it any more offensive than the complete sheltering that I grew up with -- but I do feel that she needs context to understand it. Do I fear the discussions that it would generate?
Perhaps she's not old enough to handle these topics, but if I'm completely honest I have to admit that she's most definitely heard of these things at school, on TV, etc. Pretending that they don't exist doesn't work... banning them just drives it underground (she'd read it anyway and not tell me -- I know from my own experience that you can't keep kids away from things)... so far I've been very fortunate, she's very willing to talk to me about just about anything and usually takes my advice. I believe that my openness to just lay it out on the table and give her the responsibility has a lot to do with it. As an example, she wanted to get a gauge earring (the kind that stretches the earlobe and creates a big hole). Her mother and grandmother yelled, demanded, etc, and she still bought it, "but it's the smallest size!". I basically told her that styles change, people change, and it's a permanent alteration that she can't undo. I told her that I'm OK with her trying to express herself with her clothes and hair, but that I thought that drawing the line at things that can't be undone was a reasonable request. She immediately agreed and got rid of the earring. Her grandmother (her mom's mom) was surprised and telling me how amazing it is that she listens to me, but not anybody else.
Anyhow, I'm just spewing my thoughts here and its' not very coherent... :-) How do/did other parents here deal with these issues (those related to books, I went on a tangent at the end) with their children?