Books by Aquinas
Best Online and Print Editions for Citing and Reading
Aquinas Online
Corpus Thomisticum has most of Aquinas's works in searchable Latin, though mostly in old editions. It also has a search engine for Aquinas's complete works (Index Thomisticus) a list of the Leonine editions published so far with links to those available online.
Most of Aquinas's main works were available in Latin-English facing columns on dhspriory.org/thomas. These were removed for copyright reasons, but were reuploaded to isidore.co/aquinas and to jonhaines.com/thomas. The first of these sites, Isidore.co/calibre also has an eclectic collection of PDFs, including many of books by later scholastics and twentieth-century Thomists. The Summa theologiae is also available online in English at New Advent, though the isidore version is easier to navigate.
The Aquinas Institute is working on producing Latin-English facing print volumes of Aquinas's complete works. Many of these are available online, but often not yet edited on aquinas.cc, which includes a helpful search feature.
Best Editions of Aquinas's Works
For a comprehensive catalogue of Aquinas's works and the best available editions of them, see the appendix to Jean Pierre Torrell, Saint Thomas Aquinas: The Person and His Work, 3rd ed. (Washington, DC: The Catholic University of America Press, 2023). Torrell's work also includes a chronology of Thomas's life and works. This is significantly updated from the 2nd edition. An older catalogue made by I.T. Eschmann is also available in Étienne Gilson's The Christian Philosophy of St. Thomas Aquinas, trans. L.K. Shook (Notre Dame: University of Notre Dame Press, 1994).
In general, if there is no critical edition of a work by St. Thomas—as is the case with his commentary on the Divine Names or his Quaestiones disputatae de potentia Dei—the best edition to cite is the Marietti edition, published in Turin.
Most of the critical editions of Aquinas's works have been produced by the Leonine commission (1882–). A list of the works published so far is available on Corpus Thomisticum. These can be cited by volume, page, and line number as follows: Aquinas, De ente, c. 1 (Leon. ed., 43:369:1–5). Not all the volumes in the Leonine edition are critical. For example, the original Leonine edition of the commentaries on Peri Hermeneias and on the Posterior Analytics (vols. 1/1 and 1/2) were not critical though subsequent critical editions have replaced them (1*/1 and 1*/2). The Summa theologiae and Summa contra Gentiles are not critical, but do include Thomas de Vio Cajetan's complete commentary on ST and Francis Silvestri of Ferrara's commentary on ScG. Many of the older editions are available on Archive.org. These are highlighted on Corpus Thomisticum.
Critical editions outside the Leonine Opera omnia have been made of his unfinished Roman commentary on the Sentences (the Lectio romana) and of his Liber de causis.
Lectura romana in primum Sententiarum Petri Lombardi, ed. Leonard Boyle, John Boyle (Toronto: Pontifical Institute of Mediaeval Studies, 2006).
Super librum de causis expositio, ed. H.D. Saffrey (Fribourg: Société Philosophique, 1954).
The best edition of the Sentences is still:
Scriptum super Libros Sententiarum Magistri Petri Lombardi Episcopi Parisiensis, ed. Pierre Mandonnet, Maria Fabianus Moos, 4 vols. (Paris: Sumptibus P. Lethielleux, 1929–1947).
There is no critical edition. The Aquinas Institute is working on a new Latin-English facing edition, part of which has already been published and can be purched at the St. Paul Center.
Most Convenient English / Latin Editions for Reading
For other, more complete bibliographies of the editions of Aquinas's works, see:
Thomas Aquinas in English | A Bibliography
In General
For academic work, you should always cite the best Latin edition of Aquinas's writings (see above). With that said, it is okay to also cite an existing print translation. You should never cite the online translations or online Latin texts, such as Corpus Thomisticum and Isidore.co/aquinas (see below), however useful these may be in the actual research process. The list of editions and translations below is far from complete.
With that said, the editions you should cite in academic work are usually prohibitively expensive and, in the case of the Leonine edition, too large to lug around. For the most part, then, you'll have to read Aquinas in some other edition. In general, the best edition to read is the new Aquinas Institute Opera omnia. This series of Aquinas's complete works is still in production, however, so there are gaps in what is actually available. These are well-designed 8 1/2– 11 blue, faux leather volumes with Latin-English facing columns. The editing is not always great, so you have to be wary. But they have large margins for notes and are very readable and reasonably priced (~$50–60 / volume).
Aristotelian Commentaries in English
Aquinas's commentaries on Aristotle's Physics, Nicomachean Ethics, Posterior Analytics, Metaphysics, De anima and On Interpretation were published in English translation (no Latin) in the Dumb Ox series by St. Augustine Press. Unfortunately these have pretty small margins, so they are not great for taking notes. The Posterior Analytics commentary translation is not the best one available, in my opinion, but, for the introductory essay and notes by Richard Berquist, it is worth owning the book. James Weisheipl, OP's translation of the Posterior Analytics (Magi Books, 1970) is better since it leaves a lot of technical Latin terms untranslated. Some of the translations in the Dumb Ox series, such as John Oesterle's In On Interpretation translation and Kenelm Foster's In De anima translation, are also available from other publishers. I'm not sure if the Dumb Ox version replicates the notes. In general, I would opt for buying the Aquinas Institute edition if it is available over the Dumb Ox one since it includes the Latin.
Aquinas's commentary on On Sense and What is Sensed and On Memory and Recollection was published in English translation by The Catholic University of America Press (no Latin).
Other Philosophical Commentaries
Aquinas's commentaries on the Liber de causis (Book of Causes) (trans. Richard Taylor) and on De hebdomadibus (An Exposition of the On the Hebdomads of Boethius) (trans. Schultz and Synan) were published in English translation by The Catholic University of America Press. The Liber de causis translation does not include the original Latin, but the In De hebd. translation includes the Latin from the Leonine edition. Both have great introductory essays and are very well edited and arranged. Aquinas's commentary on Boethius's De trinitate was published by the Pontifical Institute of Mediaeval Studies in two volumes in an English translation by Armand Maurer under the titles: Faith, Reason, and Theology (qq. 1–4) and The Division and Method of the Sciences (qq. 5–6). Maurer is a very skilled translator and these works include detailed notes, but not the original Latin.
Aquinas's commentary on Pseudo-Dionysius's Divine Names was recently translated by Michael Augros (Thomas Aquinas College) and published in a Latin-English facing edition by the Thomas More College Press.
Commentaries on Scripture
The best place to get Aquinas's commentaries on scripture is the Aquinas Institute, which includes the Latin, Greek, and English of the scriptural passages on which Aquinas comments as well as Latin-English facing columns for Aquinas's commentary itself. With that said, for a study edition with notes of Aquinas's commentary on the Gospel of John (only English), there is a three volume edition by The Catholic University of America Press.
Opuscula
For Aquinas's opuscula, besides the Aquinas Institute version, there is also a translation of Aquinas's On Being and Essence by Armand Maurer. This is a great translation and has detailed notes, but not the original Latin. Joseph Bobik also translated and wrote full commentaries on On Being and Essence (University of Notre Dame Press, 1988) and On the Principles of Nature and On the Mixture of the Elements (University of Notre Dame Press, 1998). Aquinas's treatise On Separate Substances was translated by Francis Lescoe (Saint Joseph College, 1959).
Disputed Questions
Aquinas's On Evil (De malo) was translated by Richard Regan (Oxford, 2003) and by John and Jean Oesterle (Notre Dame, 1995). His On Truth (De veritate) was translated in a three volumes by Robert Mulligan, James McGlynn, Robert Schmidt (Henry Regnery, 1952–1954). This is edition is well-formatted with large margins for notes, but it is very hard to find, and it is not a good translation anyway. Aquinas's Disputed Questions on Virtue were translated by Hause and Murphy (Hackett, 2010) and by Ralph McInerny (St. Augustine's Press, 1999). Aquinas's Quodlibetal Questions were translated by Turner Nevitt and Brian Davies (Oxford, 2019).
Latin Summas
The Leonine edition of the Summa theologiae is prohibitively expensive, hard to obtain, and absolutely enormous. You won't be throwing it in your backpack for a vacation at the cabin! While the Aquinas Institute Summa theologiae is the best for reading in general since it has Latin and English facing columns and is affordable, durable, and easy to obtain, there are a few other options which have their own advantages.
Reprints of the Leonine edition with Cajetan's commentary on US Letter-sized pages are available at http://leonine-reprints.ipsissima-verba.org/.
The Marietti edition Summa is three very thick volumes. It is the best edition—apart from the Leonine edition—for critical notes. All the Marietti editions are unparalleled in this regard. With that said, the Marietti edition is not easy to carry around since the volumes are so thick and, as it is out of print, it can be hard to obtain.
The best Latin edition to carry around is the Biblioteca de Autores Cristianos (BAC) edition, which is made up of five small volumes with small font and almost no margins.
Both of these are out of print, but you are most likely to find them at a college or seminary library book sale or, if you are willing to spend money on them, on an online bookseller, like AbeBooks (to search most online booksellers, go to bookfinder.com).
