The only books currently banned by the government are covered under the Methamphetamine Anti-Proliferation Act of 1999, which also covers information on explosives or weapons of mass destruction. (Those books with a disclaimer are often exempted.) This is not only to stop people from hurting others, but also themselves- people can easily blow themselves up trying to follow bomb-making recipes, and methamphetamine manufacture is just as deadly. The last specific book to be summarily banned by the US Government was I believe Ulysses, by James Joyce, in 1930, for obscenity. It was overturned in 1933.
Most books when they are 'banned' in the United States, are not prohibited from being published. Rather it means that a school or college has decided that it is unfit material and refuses to order it. Other times, a publisher will not print something controversial because they do not wish to be associated with it. While this limits the circulation of the work, the publishers, being private companies, have a right of refusal. One might as well say that every literary hopeful who receives a rejection slip is banned by these standards. Often, however, an author will hear that his book has been banned or rejected (whether by Harvard University, a small-town Midwestern library, or a large publishing house) and seize upon it as a marketing ploy, distorting the originators of the ban for notoriety's sake.
According to Ala.org's banned book's section- (Out of 6,364 challenges recorded between 1990-200,) Seventy-one percent of the challenges were to material in schools or school libraries.2 Another twenty-four percent were to material in public libraries (down two percent since 1999). Sixty percent of the challenges were brought by parents, fifteen percent by patrons, and nine percent by administrators....
Please note none came from government officials.
If the government really were controlling the book trade, then books such as the Communist Manifesto, Mein Kampf, and the Anarchist Cookbook would not still be in print. (The last is a perfect example- the author himself would like to see the book discontinued, you can read all about it in an article called The Repentant Anarchist.) In short, most people who hear about banned or suppressed books are actually hearing about books that no one wanted to publish or lend, for good or ill.