The Abiding Presence of the Holy Ghost in the Soul, by Bede Jarrett (New York: Cathedral Library Association, 1918). With Imprimatur. Contains the text of Encyclical Letter for Pentecost (1897) of Pope Leo XIII. Available in multiple formats at Manybooks, in somewhat fewer formats at Project Gutenberg, and in epub, fulltext and kindle formats at Internet Archive. A second copy is available at Internet Archive and Open Library.
Against Heresies [Or, Refutation and Overthrow of Knowledge falsely so-called ], by St. Irenaeus of Lyons (c.180 AD), trans. Alexander Roberts and William Rambaut; from Ante-Nicene Fathers, Vol. 1, eds. Alexander Roberts, James Donaldson, and A. Cleveland Coxe (Buffalo, NY: Christian Literature Publishing Co., 1885). Available on New Advent (rev. and ed. Kevin Knight), with active links to relevant Catholic Encyclopedia articles and Scripture passages; and on The Augustine Club. On Apologetics, Salvation, Heresy, Cosmology, Gnosticism, Divine Revelation, Bishops, Roman Primacy, Tradition, Sacraments, etc. [NB, A defense of Christianity against Gnosticism, which also shows that the early Church had a Catholic view of sacramental grace, the Roman primacy, the bishops as successors of the apostles, etc. Books I and II are somewhat difficult, so it may be better to start with Book III, which begins the summary of Christian teaching.]
And You Will Know the Truth: How to Defend and Explain the Catholic Faith, by Sebastian R. Fama. Available at Internet Archive in multiple formats. Apologetics.
The Beauty and Truth of the Catholic Church: Sermons from the German, adapted and edited, Volume IV, by the Rev. Edward Jones, with an Introduction by the Most Rev. John Ireland (St. Louis, Mo.; London: B. Herder, 1916). With Imprimatur. Available at Internet Archive [N.B., This volume collects discourses on God, Christ, and grace.]
The Call of All Nations [450 AD], by St. Prosper of Aquitaine, translated and annotated by Dr. P. de Letter, S.J. (Westminster, Maryland: Newman Press; London: Longmans, Green and Co., 1952). Ancient Christian Writers: Works of the Fathers in Translation (edited by Johannes Quasten and Joseph C. Plumpe), No. 14. [LINK(S) REMOVED motu proprio PENDING COPYRIGHT CHECK]
The Canons and Decrees of the Sacred and Oecumenical Council of Trent, celebrated under the Sovereign Pontiffs Paul III, Julius III and Pius IV, translated by Rev. James Waterworth; to which are prefixed essays on the external and internal history of the Council (London : Burns and Oates; New York: Catholic Publication Society Company) Available in various formats at Open Library and Internet Archive (Digitizing sponsor: MSN, Book contributor: Kelly Library, University of Toronto) [N.B. The index page says copyright year 1888, while the preface states that it was written in 1848.] Another edition (London: Dolman, 1848) may be read online at the Hanover Historical Texts Project (Scanned by Hanover College students in 1995).
The Catholic Doctrine of Grace, by George Hayward Joyce (London : Burns Oates & Washbourne, 1920). With Imprimatur. Available at Open Library and Internet Archive (Digitizing sponsor: University of Toronto, Book contributor: Pontifical Institute of Mediaeval Studies, University of Toronto)
The chief points of difference between the Catholic and Protestant creeds, by F. Laun. (New York: J. F. Wagner, 1915). With Imprimatur. May be read online or downloaded in multiple formats at Open Library.
Christian apologetics: a defense of the Catholic faith, by Rev. W. Devivier, S.J., edited by the Rt. Rev. S.G. Messmer. (New York, Cincinnati: Benziger Brothers, 1903). Available at Open Library.
Christ the Savior: A Commentary on the Third Part of St. Thomas‘ Theological Summa,May be read online on EWTN Library and TheSumma.info.
Divine grace; a series of instructions arranged according to the Baltimore catechism; an aid to teachers and preachers, by Edmund John Wirth (New York, Cincinnati: Benziger brothers, 1903). With Imprimatur. Available at Open Library and Internet Archive
Dogmatic Canons and Decrees: Authorized Translations of the Dogmatic Decrees of the Council of Trent, the Decree on the Immaculate Conception, the Syllabus of Pope Pius IX, and the Decrees of the Vatican Council (New York: Devin-Adair Co., 1912). With Imprimatur. Available at Open Library and Internet Archive [N.B., According to the Preface, it includes Canon Waterworth’s translation for the Council of Trent, Cardinal Manning’s for the First Vatican Council, and the translation of the Syllabus authorized by Cardinal McCabe, Archbishop of Dublin.]
Dogmatic Theology V. Soteriology: a dogmatic treatise on the redemption, by Joseph Pohle, translated by Arthur Preuss (St. Louis, Mo. : Herder, c1913). With Imprimatur. Available on Open Library and Internet Archive (Digitizing sponsor: MSN, Book contributor: Kelly Library, University of Toronto)
Dogmatic Theology VII. Grace, actual and habitual: a dogmatic treatise, by Joseph Pohle, translated by Arthur Preuss (St. Louis, Mo.: Herder, 1919). With Imprimatur. Available in PDF, Full Text and DjVu at Internet Archive (Book contributor: University of Toronto). Another copy of the 1919 edition with Imprimatur is available in PDF, Full text, and EPUB formats at Internet Archive and Project Gutenberg (Book contributor: Project Gutenberg). N.B., These are the best PDF copies online. Still another copy of the 1919 edition is available in mutiplie formats at Open Library and Internet Archive (Digitizing sponsor: MSN, Book contributor: Kelly Library, University of Toronto). The 1915 edition with Imprimatur is available in various formats at Open Library and Internet Archive.
Dominus Iesus: Declaration on the Unicity and Salvific Universality of Jesus Christ and the Church, by the Congregation for the Doctrine of the Faith, approved by Pope Blessed John Paul II June 16, 2000, issued August 6, 2000. May be downloaded in PDF format from Catholic Primer Library and read online at the Holy See. [NB, This is one of the most important doctrinal texts issued by the Holy See in contemporary times. It should be read together with Answers to Main Objections Against Dominus Iesus, the text of then-Cardinal Ratzinger’s interview with the Frankfurter Allgemeine Zeitung, published 22 September 2000, which may be read online at EWTN Library.] (Digitizing sponsor: MSN, Book contributor: New York Public Library).
Ecce Fides-Pillar of Truth, by Fr. John J. Pasquini. Available at Internet Archive in multiple formats. Apologetics.
The Glories of Divine Grace: a free rendering of the original treatise of P. Eusebius Nieremberg, S. J, by Matthias Joseph Scheeben, translated from the fourth revised German edition by a Benedictine Monk of St. Meinrad’s Abbey, Ind., with the consent of the author and the permission of the superior (New York, Cincinnati, St. Louis: Benziger Brothers; London: R. Washbourne; Dublin: M.H. Gill & Son, 1886). With Imprimatur for the original and approbation for the translation. Available at Open Library and Internet Archive.
Grace: Commentary on the Summa Theologica of St. Thomas, Reginald Garrigou-Lagrange, O.P. Dogmatic theology, Soteriology. May be read online on [LINK(S) REMOVED motu proprio PENDING COPYRIGHT CHECK]
Instruction on Infant Baptism, by the Sacred Congregation for the Doctrine of the Faith, approved by Pope Blessed John Paul II, October 20, 1980. May be read online at the EWTN Library and the Catholic Information Network.
The Internal Mission of the Holy Ghost, by Henry Edward Manning (New York: Catholic Publication Society, 1875). Available at Internet Archive and Open Library
Jesus Christ, The Bearer of the Water Of Life: A Christian reflection on the “New Age”, by Pontifical Council for Culture, Pontifical Council for Interreligious Dialogue. Available on website of the Holy See and on Catholic Primer as indexed here.
Most Apologetics Collection, from Fr. William Most. ZIP file containing 57 essays by Fr. William Most on subjects relating to apologetics. May be downloaded from EWTN Library.
A manual of apologetics, by Franz Xavier Jos. Koch, translated by Anna Maud Buchanan, ed. Charles Paul Bruehl (New York, J. F. Wagner, 1915). With Imprimatur. Apologetics. Available at Internet Archive (Digitizing sponsor: Google) and Open Library.
Paradoxes of Catholicism, by Robert Hugh Benson (New York: Longmans, Green, 1913). Available on Project Gutenberg and Internet Archive (Digitizing sponsor: MSN; Book contributor: Kelly Library, University of Toronto).
Predestination, by Reginald Garrigou-Lagrange. The text, while not yet completely digitized, is available at [LINK(S) REMOVED motu proprio PENDING COPYRIGHT CHECK]
The Sacraments explained according to the teaching and doctrine of the Catholic Church: with an introductory treatise on grace, by Arthur Devine (London: R. & T. Washbourn, 1918). With Imprimatur. Available at Open Library and Internet Archive(Digitizing sponsor: MSN, Book contributor: Kelly Library, University of Toronto)
Why God Became Man (Cur Deus Homo), by St. Anselm of Canterbury. Both the English and the Latin text may be downloaded in PDF format through the Saints’ Books index,