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Project Gutenberg has the following Free Downloadable E-Books by Saints:

Summa Theologica by Saint Thomas Aquinas:

Summa Theologica, Part I (Prima Pars)
From the Complete American Edition

Summa Theologica, Part I-II (Pars Prima Secundae)
From the Complete American Edition

Summa Theologica, Part II-II (Secunda Secundae)
Translated by Fathers of the English Dominican Province

The Confessions of St. Augustine:

The Confessions of St. Augustine

Saint Catherine of Siena:

Letters of Catherine Benincasa

Saint Teresa of Avila:

The Life of St. Teresa of Jesus

Thérèse, de Lisieux:

The Story of a Soul (L'Histoire d'une Âme): The Autobiography of St. Thérèse of Lisieux
With Additional Writings and Sayings of St. Thérèse