The aim of the list was to protect the faith and morals of the faithful by preventing the reading of heretical and immoral books....More of the list here, along with some discussion of what was not included:
The Index included a number of authors and intellectuals whose works are widely read today in most leading universities and are now considered as the foundations of science, e.g. Kepler's New Astronomy, his Epitome of Copernican Astronomy, and his World Harmony were quickly placed on the Index after their publication. Other noteworthy intellectual figures on the Index include Jean-Paul Sartre, Montaigne, Voltaire, Denis Diderot, Victor Hugo, Jean-Jacques Rousseau, André Gide, Emanuel Swedenborg, Baruch Spinoza, Immanuel Kant, David Hume, René Descartes, Francis Bacon, Thomas Browne, John Milton, John Locke, Nicolaus Copernicus, Galileo Galilei, Blaise Pascal, and Hugo Grotius. Charles Darwin's works were never included.
Not on the Index were Aristophanes, Juvenal, John Cleland, James Joyce and D. H. Lawrence. According to Wallace et al., this was because the primary criterion for banning the work was anticlericalism, blasphemy and heresy.