Each book that is banned or censored is done so for the content within the pages. There are a few common reasons that books have been banned or censored in schools, libraries, and book stores. These include:
Racial Issues: About and/or encouraging racism towards one or more group of people.
Encouragement of "Damaging" Lifestyles: Content of book encourages lifestyle choices that are not of the norm or could be considered dangerous or damaging. This could include drug use, co-habitation without marriage, or homosexuality.
Blasphemous Dialog: The author of the book uses words such as "God" or "Jesus" as profanity. This could also include any use of profanity or swear words within the text that any reader might find offensive.
Sexual Situations or Dialog: Many books with content that include sexual situations or dialog are banned or censored.
Violence or Negativity: Books with content that include violence are often banned or censored. Some books have also been deemed too negative or depressing and have been banned or censored as well.
Presence of Witchcraft: Books that include magic or witchcraft themes. A common example of these types of books are J.K. Rowling's Harry Potter Series.
Religious Affiliations (unpopular religions): Books have been banned or censored due to an unpopular religious views or opinions in the content of the book. This is most commonly related to satanic or witchcraft themes found in the book. Although, many books have also been banned or censored for any religious views in general that might not coincide with the public view.
Political Bias: Most Commonly occurs when books support or examine extreme political parties/philosophies such as: fascism, communism, anarchism, etc.
Age Inappropriate: These books have been banned or censored due to their content and the age level at which they are aimed. In some cases children's books are viewed to have "inappropriate" themes for the age level at which they are written for.
Many books have been banned or censored in one or more of these categories due to a misjudgment or misunderstanding about the books contents and message. Although a book may have been banned or labeled a certain way, it is important that the reader makes his/her own judgements on the book. Many books that have been banned or censored later were dropped from banned books lists and were no longer considered controversial. For this reason, banned books week occurs yearly to give readers a chance to revisit past or recently banned books to encourage a fresh look into the controversies the books faced.
Source: "Common Reasons for Banning Books," Fort Lewis College, John F. Reed Library. Banned Books, Censorship & Free Speech. November 15, 2013. Web. March 19, 2014.