Almost any book worth reading is a dangerous book. I once knew a young man who jumped off a roof and broke his leg after reading the Harry Potter books. It seems he wanted to test himself for latent magical abilities, like Neville's relatives did to him when he was a kid, trying to prove that he wasn't a Squib. (This young man had high-functioning Downs Syndrome; he was bright enough to read Harry Potter, but lacked a clear understanding of the line between reality and fantasy.)
Books have the power to shape our ideas, for good or ill. Speculative fiction may have even greater power because it is not bounded by the limits of the real world. Of course, not everyone who reads Harry Potter will respond the way this young man did—the vast majority will not. But the ideas contained in books—whether ideas intentionally worked into the themes of the story by the author or not—will inevitably interact with our own, and shifts in understanding can take place. Stories may challenge our beliefs, or they may confirm and solidify them—or perhaps they will affect them in more oblique ways. Either way, whenever we encounter good story-telling, it is unlikely we will walk away unchanged.
This is not a bad thing, of course. In fact, I think this is how any well-told story should be. I don't reject books because I consider them “dangerous”. In fact, I think the word could be applied to almost any of my favorite books, and certainly to the ones I recommend most. “Read this book. It will change you.” Haven't you ever recommended a book to someone that way? But because I know they can have a powerful influence on our beliefs—and by extension, how we live our lives—I do think carefully about what books I pass on to others, and under what circumstances. Some books I may recommend only to certain people, like the series I will discuss in a moment. Some I may recommend while expressing reservations or adding disclaimers. (The Twilight books come immediately to mind.) With children and teens, there are a lot of books I would prefer to share and discuss with them, rather than just setting them loose unguided. (I feel this way about much of the Bible, as well as a lot of classic works). And, though I can't think of any off the top of my head (at least not any I actually finished reading), there may be some books I wouldn't pass on to anyone under any circumstances.
Some time ago on Mark Reads I caused a bit of a stir by saying that I wouldn't recommend Philip Pullman's YA fantasy trilogy His Dark Materials to young people. Why not? In essence, I feel that the series co-opts the thrill of numinous experience through its fantasy setting and inspiring characters and utilizes this thrill for such a harshly anti-Christian and anti-God message that I cannot support putting the books in the hands of people who I feel are likely to accept that message too easily and uncritically.
I have recommended the books to adult readers, especially fellow Christians, for the purpose of examining challenges to the faith and discussing the implications of them. There's no doubt they are well-written, highly engaging books with many elements and themes that appeal to a broad audience, including young people. And I would even say that I agree with some of the themes—themes like embracing the pursuit of knowledge and sacrificing one's own immediate happiness for long-term goals and greater good. But there are a couple of points that I object to strongly, and the fact that the books are well-written and contain some positive themes make me feel all the more compelled to avoid endorsing them. It's like hiding poison in a delicious chocolate cake.
I don't support censorship. However, I think there will always be some tendency by human institutions run by humans to have a certain bias in what information we select or include in our media and communication materials. People in certain positions in a free society—librarians at public libraries, for example—have a responsibility to repress their inclinations to “select out” that which they disagree with (and maybe also to “select in” that which they wish to promote) to a degree that could reasonably be considered censorship. Were I in such a position, I wouldn't “select out” HDM from the shelves of YA fantasy fiction. There is no doubt that it belongs there.
But does that same idea apply to my position as a classroom teacher? I don't believe it does. The books I place on my classroom bookshelves are: 1) obviously not meant to be a comprehensive collection of YA books, and 2) could be viewed as receiving my personal endorsement. Therefore, I have chosen not to put the HDM books on my classroom bookshelves.
Finally, I wouldn't dream of forbidding or even discouraging a teen who is already inclined to pick up the HDM books from doing so. I'm not as dumb as Professor Umbridge, after all! I might—if in a situation where it is appropriate to offer some guidance—encourage her to search out the underlying themes and ideas Pullman might be trying to promote, and to think carefully about whether or not she agrees with them, and what other viewpoints there might be. I might even articulate some of those other viewpoints.
Censorship is not the right approach to "dangerous books". People have the right to access information and ideas--even if others dislike those ideas or the effects they might produce. However, I believe that in a free society people also have the right to make selections about what information and ideas we personally endorse.