We touched on this topic once when we were discussing YA and children's fiction. I think it would be a good idea - it's not censorship, just lets readers know what is likely to be in a book. I guess the only problem is quantifying the violence or sex or 'adult themes'.
Brett Easton Ellis's book American Psycho has an R rating in Australia and cannot be sold to anyone under 18, but like R movies, it doesn't mean under 18s can't get hold of it at home. As far as I know, this is the only book we sell/have sold that has this rating.

I have stopped reading books by authors who have given me unnecessary nasty shocks in stories which were pleasant until then. They may never do it again but I don't give them a second chance to upset me.

There was an item on the news that Amazon have had to withdraw a book about paedophilia. At least one would recognise it from the title. I'm not sure what to think about censorship of this book by public pressure. I deplore the subject but I do not approve of banning or burning books either. I would not want to read it but I imagine there would be a legitimate reason for social workers etc. to study such a work.

I didn't want to suggest censorship, but rather fair warning to the average reader.
However, it might not be a bad idea to have some type of rating on books that appeal to YA...such as the Twilight series....to alert parents to questionable material. Then they can make intelligent decisions for their own children.
The movie ratings seem to be able to generally identify offensive content satisfactorily.
Again, this would be an alert for discerning readers, not an attempt to prevent anyone from buying or reading the book.

Why is it acceptable to flag movie and music recording content, but we get squeamish about identifying book content?

We get squeamish about identifying book content because books are so important, and this is very subject to abuse and misinterpretation. I admit that I don't want to read that kind of thing either. If you're not sure about a new author, perhaps reading reviews in advance could be helpful. Publishers marketing the book through cover art and blurbs give good clues as to who they and the author are trying to attract. And if all this fails (it does), you can always close a book that disturbs you (I have) and just decide not to read that author again.

Rosefolly

I have just finished "The Bad Book Affair" by Ian Sansom which touches on the subject of questionable books which can be borrowed by YA readers.
If you get the same type of paperback which I borrowed, do not try to wipe off the coffee stain. It is deliberate!
I always clean up bookcovers if I am going to read them in bed. This one fooled me:-)

If a minor tries to rent a CD which is rated R, are they allowed to do this?
If accompanied by adult.....and so goes the world.
Caveat Emptor !
I feel that this whole topic makes most feel flummoxed.
After posing the question, I find that I am more conflicted than ever.
Add that to my list of "imponderables" LOL

Sorry, I am not conflicted. My two pennies... or perhaps a few cents more than that:

While I understand the desire to know ahead of time if this is a book that would please a person, I also think that in some ways, this goes against the very reason we read. We read a blurb on the cover, a summary in a book mag, or hear snippets from a friend and then choose to read the book to be taken on a journey. The journey is not always pleasing to us but if we knew what to expect in advance, why bother to read the book at all? If we knew every mystery ended with "the butler did it", would we bother to read the book? Of course not. I know this is not apples to apples... or is it? We read to be surprised, to be enlightened, to escape, to be entertained... and that means we cannot know the details in advance, the good details, the bad details nor the ugly details. Rating a book for any reason would, in my apparently not-so-humble opinion, give it away in either part or whole. And that spoils the surprise.

PAM

How about this --

When you're buying a book by an author you've never read before, buy it from a store w/ a return policy. That way, if you don't like it you can return it. Nothing is lost.

I found this and think it would be a great resource:

Here is a link that might be useful: Finding award-winning books for YA