Amy Welborn asked for reading recommendations for her 14 year old daughter; to date, she's had about fourteen dozen suggestions, including not only every book I might have recommended, but practically every historical fiction and fantasy novel I've ever read.
The one recommendation I absolutely cannot agree with was this:
Sooner or later she's going to tangle with Ayn Rand, and she should - anyone who is going to lead life guided by a philosophy ought to understand competing philosophies in their intellectual environment, and understand where they agree and disagree, and why.
As I replied in the following comment, all you need to know about Ayn Rand is that she was bull goose looney; objectivism sort of dissolves under the weight of its own bile, and there are better things to do with your time than acquaint yourself with the locus classicus for tripe like that.
But it does give me an idea for a book I think would be very helpful for teenagers: The Not-So-Great Ideas, a compendium of bum philosophies that captivate nineteen-year-olds when they first encounter them. Objectivism, solipsism, Marxism, materialism, nihilism: the dead-end sinks of human thought and sources of human misery. Collect them in a single book for high school students that, if nothing else, will teach them their parents aren't the only grown-ups who have lousy ideas.