We highly recommend a digital library Internet Archive, where a number of books is available for free, including those concerning Aquinas’ thought. Although these are usually older publications, many of them still constitute very valuable resources for studies on St. Thomas’ theology and philosophy. Among them are (alphabetically):
1. Barron Robert E., Thomas Aquinas. Spiritual Master, Crossroad Publishing Co. 1996;
2. Carr Henry, The Function of the Phantasm in St. Thomas Aquinas, in: Philosophical Essays Presented to John Watson, Queen’s University 1922, p. 179-203;
3. Copleston Frederick, Aquinas, Penguin Books 1975;
4. D’Arcy M.C. SJ, Thomas Aquinas, Ernest Benn Limited 1930;
5. Grabmann Martin, Thomas Aquinas, His Personality and Thought, Russell & Russell 1963;
6. Henle Robert J., Saint Thomas and Platonism, Martinus Nijhoff 1970;
7. Kenny Anthony, Aquinas, Oxford University Press 1980 (we also recommend a BBC talk of Bryan Magee and Anthony Kenny on medieval philosophy, especially on Aquinas’ thought);
8. McInerny Ralph M., Boethius and Aquinas, The Catholic University of America Press 1990/2012;
9. McInerny Ralph M., St. Thomas Aquinas, University of Notre Dame Press 1982;
10. Petitot Hyacinthe OP, The Life and Spirit of Thomas Aquinas, Priory Press 1966;
11. Wicksteed Philip H., Dante & Aquinas, J.M. Dent & Sons Ltd. 1913.
It is worth to register on the website (it is free), because some books are available only for a digital “loan” (publications marked as „Borrow” and „Join waitlist”), possible only after log in.