The following is a list I compiled of free English-language Catholic e-books from the Internet Archive. All books can be downloaded in the PDF format.
Catholic doctrine and apologetics
Francis X Doyle SJ: Defense of the Catholic Church (1927) (536 pages, 26.4 MiB) link
Fr. Sylvester Berry: The Church of Christ: an apologetic and dogmatic treatise (1927) (592 pages, 33.8 MiB) link
Francis Patrick Kenrick: The primacy of the apostolic see vindicated (1855) (454 pages, 35.5 MiB) link
Rev. Thomas Slater SJ: Questions of moral theology (1915) (438 pages, 39.2 MiB) link
George Hayward Joyce: The question of miracles (1914) (157 pages, 2.2 MiB) link
Charles Coppens SJ: A Brief Text-book of Logic and Mental Philosophy (1891) (296 pages, 11.2 MiB) link
Bernard O’Reilly: Beautiful pearls of Catholic truth (1897) (660 pages, 64 MiB) link
Sylvester J. Hunter: Outlines of Dogmatic Theology, 3 volumes (1895)
Volume I (563 pages, 6.9 MiB) link
Volume II (633 pages, 10 MiB) link
Volume III (529 pages, 6.4 MiB) link
Fr. Michael Müller 9-volume series “God the teacher of mankind” (1877)
Volume 1: The Church And Her Enemies (621 pages, 18.2 MiB) link
Volume 2: The Apostles’ Creed (585 pages, 15.2 MiB) link
Volume 3: The Greatest And The First Commandment (753 pages, 19.1 MiB) link
Volume 4: Explanation Of Commandments, Continued) (507 pages, 15.7 MiB) link
Volume 5: Dignity, Authority, And Duties Of Parents, Ecclesiastical And Civil Powers) (848 pages, 46.4 MiB) link
Volume 6: Grace And The Sacraments: Baptism, Confirmation, Extreme Unction, Holy Orders And Matrimony (609 pages, 16.9 MiB) link
Volume 7: Holy Sacrifice of the Mass) (669 pages, 22.9 MiB) link
Volume 8: The Sacraments Of The Holy Eucharist And Penance (614 pages, 17.6 MiB) link
Volume 9: Sacramentals, Prayer, Vices And Virtues; Christian Perfection, Etc. (512 pages, 29.5 MiB) link
Prayer and spiritual life
Fr. Michael Müller: Prayer, the Key of Salvation (1868) (279 pages, 5.4 MiB) link
Adolphe Tanquerey: The spiritual life a treatise on ascetical and mystical theology (1930) (827 pages, 52.0 MiB) link
Fr. Francis Xavier Lasance: The Prisoner of Love: instructions and reflections on our duties towards Jesus in the most Holy Sacrament of the Altar (1918) (532 pages, 24.1 MiB) link
Édouard de Lehen SJ: The way of interior peace (author died 1867, published in 1889) (388 pages, 33.0 MiB) link
Henry Churchill Semple SJ: Heaven open to souls (1916) (576 pages, 45.9 MiB) link
The unchangeable church, her heroes, her martyrs, her trials, and her triumphs (1906) (Volume I: 502 pages, 35.9 MiB, link) (Volume II: 514 pages, 36.3 MiB, link)
Catechisms and catechism commentaries
Catechism of the Council of Trent for parish priests (translated by John A. McHugh OP and Charles J. Callan OP) (668 pages, 31.5 MiB) link
Rev. Francis Spirago: The Catechism Explained (1927) (754 pages, 82.4 MiB) link
Rev. Francis Spirago: Anecdotes and examples illustrating the Catholic catechism (1904) (644 pages, 35.4 MiB) link
Other
Fr. Michael Müller CSSR: The church and her enemies (1880) (614 pages, 39.1 MiB) link
Rev. Fr. Francis Hunolt: Sermons on Our Lord and His Blessed Mother Volume II (1897) (534 pages, 42.8 MiB) link
Books by bishops and cardinals
Cardinal Edward Henry Manning: The present crisis of the Holy See tested by prophecy (1861) (116 pages, 2.9 MiB) link
Cardinal James Gibbons: The Faith of Our Fathers (1917) (450 pages, 2.5 MiB) link
Cardinal James Gibbons: The Light Of The Cross In The Twentieth Century Volume 3 (1905) (375 pages, 17.0 MiB) link
Bishop Richard Challoner: Considerations upon Christian truths and Christian duties, digested into meditations for every day in the year (died in 1781, this edition published in 1874) (616 pages, 22.8 MiB) link
Bishop John S. Vaughan: Life after death (1923) (256 pages, 7.9 MiB) link
Saints
St. Alfonsus Ligouri: The complete works of Saint Alphonsus de Liguori : the ascetical works: Volume 5 The Passion And The Death Of Jesus Christ (published 1887) (521 pages, 12.6 MiB) link
St. Alfonsus Ligouri: Preaching (published 1890) (596 pages, 28.7 MiB) link
Msgr. Bougaud, Bishop of Laval: Revelations of the Sacred Heart to Blessed Margaret Mary, and the history of her life (published 1890) (418 pages, 23.4 MiB) link
