>>The censorship is not typically done by the U.S. government (except Ulysses), but by businesses or religions organizations (where the book is negative toward them) or school systems in which case it is done by over zealous parents via the PTA. The many faces of the U.S.... Now as to subversive information against the U.S. (and how to make weapons that could be used against the U.S. et al) that is a different story and the FBI watches that closely.
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>To what extent can religious organizations ban a book? How far does that go?

Here, they usually convince local businesses not to carry it or they will be boycotted. They can also convince librarian to remove it from shelf, by applying political pressure. Pressure can only be applied to a business, if business feels that most of it's customers belong to and will listen to the leader of the religious organization.