From TIME Magazine:
Since 1982, the American Library Association has sponsored Banned Books Week to pay tribute to free speech and open libraries. The tradition began as a nod to how far society has come since 1557, when Pope Paul IV first established The Index of Prohibited Books to protect Catholics from controversial ideas. Pope Paul VI would abolish it 409 years later, although attempts at censorship still remain.
Here are some of the most challenged books of all time.