Authors: A

Albert the Great, O.P., (Albertus Magnus) (Albert, Count von Bollstaedt); Saint, Doctor [1206 - 1280]

On Cleaving to God (wiki)
On Cleaving to God (.html)
On Cleaving to God (.pdf)

Anselm, Saint, Doctor [1033-1109]

The Devotions of Saint Anselm (wiki)
Cur Deus Homo(.pdf)
Cur Deus Homo(wiki)
Proslogium, Monologium, (Including In Behalf of the Fool by Gaunilon and Reply by Anslem) (.pdf)
Proslogium (wiki)

Aquinas, O.P., Thomas; Saint, Doctor [1225 - 1274]

Catechetical Instructions (wiki) - A short Catechism of the Catholic Church; providing a discussion of the Ten Commandments, The Lord's Prayer, The Sacraments of the Catholic Church, and the Hail Mary.
Catechetical Instructions (.html)
Catechetical Instructions (.pdf)
Summa Theologica (.html) - Perhaps the most definitive work on the subjects of Theology and Faith, the Summa Theologica has profoundly impacted Christian thought and philosophy; not only Catholic, but protestant as well.
Companion to the Summa - Walter Farrell, O.P.
The Summa Contra Gentiles of St. Thomas Aquinas (Abridge) (.pdf) - (Of God and His Creatures: an Annotated Translation by Fr. Rickaby, S.J.)
The Summa Contra Gentiles, Book 1 (Unabridged) (.pdf) - (Translated by the English Dominican Fathers)
The Summa Contra Gentiles, Book 2 (Unabridged) (.pdf) - (Translated by the English Dominican Fathers)
The Summa Contra Gentiles, Book 3, Volume 1 (Unabridged) (.pdf) - (Translated by the English Dominican Fathers)
The Summa Contra Gentiles, Book 3, Volume 2 (Unabridged) (.pdf) - (Translated by the English Dominican Fathers)
The Summa Contra Gentiles, Book 4 (Unabridged) (.pdf) - (Translated by the English Dominican Fathers)
The Summa Contra Gentiles of St. Thomas Aquinas (Unabridged) (wiki) - The English Dominican Fathers
Catena Aurea: The Gospel of Matthew (.pdf) - A commentary on the Gospel by St. Thomas Aquinas
Catena Aurea: The Gospel of Matthew (Revised) (.pdf) - A revised version with improved navigation (5.5 MB)
Catena Aurea: The Gospel of Matthew (.html) - Online version in a chapter format
Catena Aurea: The Gospel of Mark (.pdf) - A commentary on the Gospel by St. Thomas Aquinas
De Aeternitate Mundi (.pdf) - "On the Eternity of the World" by St. Thomas Aquinas
De Aeternitate Mundi (wiki)
De Ente et Essentia (.pdf)- "On Being and Essence" by St. Thomas Aquinas
De Ente et Essentia (wiki)

Athanasius, Saint, Doctor [ca 296-373]

On the Incarnation (wiki)

Athenagoras, [second century Apologist]

A Plea for the Christians (wiki)

Aristotle, Pagan Greek philosopher, [384 - 322 BC]