[Cúdae]

First, I must admit you scared me a little by quoting me! But that aside, I will give my answers to your questions, as well as some ironic background jibberish.

1. I think that this will interest you: Ray Bradbury was banned because of Farenheit 451. The school officials felt the book was too "extreme for the minds of young readers" and also that "Bradbury's writings in general were likely to corrupt young faithful minds."
Is it just me, or does this make it even more ironic?

2. To the first question: No, there is no <u>valid</u> knowledge, just knowledge that is believed by some to be valid and therefore put to use for the good of the many.
To the second question: What it means when one author is banned and another is not is that the valid knowledge for some was put to use for the good of the many but only in certain areas. That confused even me so let me explain in...er...plainer English. Harry Potter is incredibly popular, but it has things that some people find objectionable by the standards of their own valid knowledge. The chief of these is magic. Tolkien's works also have things that people find objectionable, again by the standards of their own knowledge. The chief of these is either magic or some form of over-analyzed heresy (or as I like to call it: The Anit-Tolkien Yadda-Yadda). By over-analyzed I mean that the words were taken too litrally. Say I typed "The sky seems to be stained with blood" referring to the sky in the morning before a storm. If you take that as someone just reading it says "The sky is red." If you take it as someone far more interested in what could possibly be wrong it might say "This person is about the commit murder. It is a prophetic sentence."
Another thing that comes into this answer to your question: A person may reasonably spread his opinion, formed by his own valid knowledge, and get any book banned. All this person would need is a little persuasion.
So in straight answer: When one author is banned and another is not, it simply means that one person used the knowledge they possess to form an opinion with reasons to back it up then used his own power of persuasion to ban the author.
Factor Ray Bradbury in? Easy. Someone found something he wrote objectionable, backed up their opinion, spread their opinion, persuaded, and finally got their way.

I am not going to start in on whether this is a good thing or a bad thing.