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Adult books are generally classified as fiction or non-fiction. Adult fiction will generally begin with an F, followed by part of the author's last name. If it's genre fiction, the library catalog will indicate the collection where it is shelved: mystery, fantasy, romance, western, and so forth.
Adult non-fiction usually has a call number and part of the author's last name.
Everyone knows this stuff. I'm not mentioning it because I think people don't know their way around a library. I mention it because BOOKS DON'T COME WITH MPAA RATINGS, like movies do. (Even TV and music have a ratings system.)
When you let your child loose in the library (at any age), or when you yourself roam the stacks of books, remember that, beyond the call number classification, books don't come with a ratings system. And especially remember this: books located in the adult books section of the library are ADULT BOOKS. That doesn't mean their content may not be suitable for younger readers, only that you shouldn't ever assume that it is. Those books weren't necessarily written with a young reader—and a young reader's sensibilities—in mind.
It is good practice for you to quickly check through any book you or your child selects. Check the call number. Read the back copy. Skim a few pages. A book that may be geared for young adults may not be suitable for your particular teen. Even a children's book may include content that your child may not be developmentally and emotionally prepared to handle. Only you have the real expertise to assist your child when selecting books to read.
And always, always, look for appropriateness if the book your child or teen picks up is from the adult stacks. That's my parental two cents worth, anyway.
(I've included the link to the MPAA ratings web page below, if you're curious to see what thematic elements and contents generate which ratings in movies.)
http://www.mpaa.org/ratings/what-each-rating-means