As far as basic introductions to St. Thomas Aquinas go, any reader will find this small book a thought-provoking, yet very accessible read. Aquinas is not an easy read, and one often needs a good, solid guide. One finds such a person in the likes of Peter Kreeft, Thomas Joseph White, and of course, Edward Feser...
...and Robert Barron.
One of the reasons why a good, initial guide to Aquinas is necessary is that it's easy to find him dry, or if not, to get lost in the weeds. There's also an ability to read him wrongly. As Feser so aptly points out in Aquinas, what can often be a stumbling block to reading him-- such that he is often wrongly dismissed-- is because the modern reader often doesn't know what Aquinas actually means by certain terms. The basics of medieval metaphysics are therefore lost on us, and so we need an initiation.
Another way in which we moderns are apt to read Aquinas wrongly, Barron writes, is that we tend to separate Aquinas's spirituality from his theology when that separation would have made no sense to Aquinas. His spirituality is his theology and vice versa, and it-- and Catholicism-- has metaphysical import. How we do or don't understand reality is related to how we do and don't understand what we-- and St. Thomas-- mean by "God."
What Barron excels at is why Aquinas matters not just for academic theologians and philosophers-- or geeks-- but also for the average believer. He gives us an Aquinas who is rigorous but far from "dry," and he gives us an idea of why we should want to engage and allow ourselves to be engaged by him. His Aquinas is a "master of the sacred page," steeped in the reading and interpretation of Scripture; Thomism, therefore, isn't just philosophy. Moreover, toss out Thomas, and you toss out a powerful evangelical guide: Aquinas, as Barron recently related to Matt Fradd on the podcast Pints with Aquinas, isn't for everybody. But don't underestimate his ability to appeal to people. Aquinas appealed to the young Barron at fourteen, along with countless others-- because some people just are more reachable that way.
Also important in Barron's treatment of Aquinas is reading the Summa Theologiae with an eye toward practical spirituality. Other than to say that God is "the sheer act of To Be, itself," Aquinas's theology is more about who and what God is not. The reason is because he knows that we sinners commonly try to make God into something that he is not, all the more so that we might bracket him or avoid him, if not reject him outright, which are really a rejection of obedience that comes with the Fall. Thomas respects Reason as a gift from God-- which also means that he's a realist. He is well aware of the reality that all gifts can be misused or abused, because the human mind is fallen. So with God, we tend to grasp or hide. To reject who and what God is not (and this is a good link-up to a book like Ulrich Lehner's God is Not Nice) is crucial if we say we want a relationship with God. Because it means letting God be himself, as we ought to do and try to do with anyone we say we love. Idolatry would mean the opposite. Barron's approach to the Summa is to outline the common moves a sinner makes regarding God, which is really very helpful.
In short, this short book is worth reading, worth contemplating. Just get the recent edition, though. The earlier edition-- as Barron points out-- referred to God as "he," only to refer to him a couple of paragraphs afterward as "she." That got a bit weird, not least because Barron didn't and doesn't strike me as the kind of person who would make that sort of move. But correcting for it and Barron's reassurance on Amazon.com that these discrepancies aren't present in the later, more recent edition are why they shouldn't take away from the overall substance of the book.
Thomas Aquinas: Spiritual Master has several potential uses: can be helpful for spiritual directors. But also, if remaining attentive to the various "sinner's moves" that Aquinas identifies, whose implications Barron conveys in clear, thoughtful language, it can be a useful guide for Confession, too. Way too often, we name our sins like they're a laundry list based on the Decalogue, the Beatitudes, and the Precepts of the Church. But do we think about why we commit them, our motivations, and the kinds of moves and excuses we make? To ask those questions and probe ourselves for answers is to think about why the Decalogue, etc. exist at all.