In the context of Counter-Reformation, explain the Index Librorum Prohibitorum of the Councils and documents.

The 1559–1967 Index Librorum Prohibitorum was a directory of prohibited books which was updated twenty times during the next four centuries as books were added or removed from the list by the Sacred Congregation of the Index. It was divided into three classes. The first class listed heretical writers, the second class listed heretical works, and the third class listed forbidden writings which were published without the name of the author. The Index was finally suspended on 29 March 1967.
Index Librorum Prohibitorum Sacred Congregation of the Index