Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances. - See more at: http://constitution.findlaw.com/amendment1.html#sthash.Nh0el9m9.dpuf
Timeline of Banned Books: Lists time and reason for banning books
ALA: American Library Association links for censorship