To understand St. Thomas better, it helps to have some background knowledge of the metaphysical structure of thought he inherited and developed. As a starter, I suggest:
Edward Feser, The Last Superstition, St. Augustine’s Press.
I’ve mentioned this book before as a robust refutation of the so-called “New Atheism”. It’s also a very good introduction to the whole stream of Aristo-Thomist thought through the centuries.
Once past this introduction, this forthcoming book from Feser is likely to become the primary recommendation.
Edward Feser, Scholastic Metaphysics, Editiones Scholasticae. (ISBN 978-3-86838-544-1).
The following three books give excellent presentations of the range of Aristotle’s philosophy.
Christopher Shields, Aristotle, Routledge.
Jonathan Lear, Aristotle, the Desire to Understand, Cambridge University Press.
Jonathan Barnes (ed.), The Cambridge Companion to Aristotle, Cambridge University Press.
Although neglected in modern times, Aristotle’s thought is not dead! If you want to see a thoroughly modern presentation of where Aristotelianism is today, try the following:
David Oderberg, Real Essentialism, Routledge.
It's well worth looking out some of the older treatments of Aquinas's philosophical system. Some have to be found second hand or on Google Books, but an increasing number of re-prints are becoming available. Examples of this type of literature are:
A.D. Sertillanges, Foundations of Thomistic Philosophy, in the Catholic Library of Religious Knowledge series published by Sands & Co./Herder Co.
R.P. Phillips, Modern Thomistic Philosophy, Vol 1. The Philosophy of Nature and Vol. 2 Metaphysics, Burns, Oates & Washbourne ltd.
Henri Renard, The Philosophy of Being, Kessinger.
More general than these, the first is a gentle introduction, the second a compact sophisticated survey of everything, very useful as a reference.
Daniel J. Sullivan, An Introduction to Philosophy, Tan Books.
William A. Wallace, The Elements of Philosophy, St. Pauls.
The so-called “Neo-Scholastic” movement of the late nineteenth into the middle of the twentieth century faded into obscurity after the second Vatican Council as the flights of fashion favoured other approaches to the metaphysical foundations of theology. As the weaknesses of such foundational approaches have been more clearly understood, the “classical” Aristo-Thomism of the neo-scholastic movement is regaining the attention it deserves. One figure in particular seems to be gaining favour: Reginald Garrigou-Lagrange. Here are two books of his: the first is a thorough-going summary of neo-scholastic Aristo-Thomistic metaphysics (and more); the second an extended exposition of natural theology (so making up a commentary on the first few questions of the summa). These are not easy going!
Reginald Garrigou-Lagrange, Reality: A Synthesis of Thomistic Thought, Ex Fontibus.
Reginald Garrigou-Lagrange, God: His Existence and His Nature (2 volumes), Ex Fontibus.
I haven’t covered Plato or neo-Platonism or the stream of thought that comes from Augustine in this post. That will have to follow!