Books can be immensely powerful. The ideas in them can change the way people think. Yet it was the Nazis and Stalin's officers who committed terrible crimes, and not Mein Kampf or the Communist Manifesto - and of course, the Manifesto contained many key ideas that are still relevant and important today, long after Stalin has gone. There is a crucial distinction between the book and its effect - it's crucial because if you talk about a book being harmful rather than its effect you begin to legitimise censorship. Abhorrent ideas need to be challenged by better ones, not banned.