Summa Theologiae
St. Thomas Aquinas

This beginners' textbook in theology, never finished by
St. Thomas Aquinas, sets out to summarize the entirety of
sacred theology.  Used as the primary teacher of sacred
theology in seminaries for centuries, it had the unique
privilege of being kept open on the altar during the Council
of Trent, so that the fathers of the Council could always
and immediately draw from its wisdom, the only book besides
the Sacred Scriptures themselves accorded this honor.  The
great Leo XIII, of happy memory, stated in his encyclical
Aeterni Patris that St. Thomas Aquinas should be the
basis for any Catholic philosophical and theological
education, and this is the greatest of St. Thomas's works.
Its value cannot be overstated.
Prima Pars (PDF) (free)
Prima Secundæ (PDF) (free)
Secunda Secundæ (PDF) (free)
Tertia Pars (PDF) (free)