The Index Librorum Prohibitorum (Index of Prohibited Books)—also called Index Expurgatorius—is a list of publications which Roman Catholics were banned from reading, "pernicious books", and also the rules of the Church relating to books. The aim of the list was to prevent the reading of immoral books or works containing theological errors and so prevent the corruption of the faithful.
It was created in 1559 by the Sacred Congregation of the Inquisition of the Roman Catholic Church (later the Congregation for the Doctrine of the Faith). The index was regularly updated until the 1948 edition, with materials being added by either the Congregation or the Pope. The list was not simply a reactive work; the authors were encouraged to defend their works, they could re-publish with elisions if they wished to avoid a ban, and pre-publication censorship was encouraged.
Some of the Index's actions were of a definite political content: in 1926, the Action Française magazine, espousing far-right French causes, was put in the Index.
The Index's effects were felt throughout much of the Catholic world. For many years in areas as diverse as Quebec and Poland it was very difficult to find copies of indexed works, especially outside of major cities. The index as an official list was relaxed in 1966 under Pope Paul VI following the end of the Second Vatican Council and largely due to practical considerations. It remains a sin for Catholics to read books which are injurous to faith and/or morals.
"Index of Prohibited Books", The Catholic Encyclopedia, 1913 (http://www.newadvent.org/cathen/07721a.htm): "The first Roman Index of Prohibited Books (Index librorum prohibitorum), published in 1559 under Paul IV, was very severe, and was therefore mitigated under that pontiff by decree of the Holy Office of 14 June of the same year. It was only in 1909 that this Moderatio Indicis librorum prohibitorum (Mitigation of the Index of Prohibited Books) was rediscovered in "Codex Vaticanus lat. 3958, fol. 74", and was published for the first time." The effectiveness of such a mitigation is open to question.
Prohibitions made by other congregations (mostly the Holy Office) were simply passed on to the Congregation of the Index, where the final decrees were drafted and made public, after approval of the Pope (who always had the possibility to condemn an author personally--only a few examples, such as Lamennais and Hermes).
The Congregation of the Index was abolished in 1917, when the rules on the reading of books were again reelaborated in the new Codex Iuris Canonici.
"Index of Prohibited Books", The Catholic Encyclopedia, 1913: "The first Roman Index of Prohibited Books (Index librorum prohibitorum), published in 1559 under Paul IV, was very severe, and was therefore mitigated under that pontiff by decree of the Holy Office of 14 June of the same year.
The new index, published at length by Clement VIII (1596), contained, besides additions to the catalogue of forbiddenbooks, not only the ten rules but, directly after them, an instruction on the prohibition, expurgation, and printing of books, some remarks on the fourth and ninth rules, and on several of the forbiddenbooks.
Regarding them paragraph 11 says: All books are forbidden that insult God or the Blessed Virgin Mary or the saints or the Catholic Church and her rites, the sacraments or the Apostolic See.
From the previously mentioned arrangement of all forbiddenbooks in three groups it clearly follows that the Church not only keeps within the limits of her right, but also forbids only as much as she is bound to forbid by reason of her office as teacher and guide of all the faithful.