Reddit reviews Introduction To Saint Thomas Aquinas

Robert Royal's book A Deeper Vision is a great history/summary of all major 20th century Catholic thinkers. It's probably especially useful for you because it does a good job of situating recent Catholic thinkers within the context of secular philosophy.

Augustine and Aquinas are going to the Big Titans you want to get through before anyone else, and I'm going to third the need for an understanding of Aristotle, but also toss out that some Plato might be useful too, for an understanding of stuff that Aristotle is building on/rejecting, and later stuff that Augustine is building on/rejecting.

From what I remember of my undergraduate, this wasn't a bad intro to Thomas: https://www.amazon.com/Introduction-Saint-Thomas-Aquinas/dp/0075536536/ref=sr_1_3?s=books&ie=UTF8&qid=1468082922&sr=1-3&keywords=introduction+aquinas

And Maritain's Intro to Philosophy is probably also helpful, if dense: https://www.amazon.com/Introduction-Philosophy-Sheed-Ward-Classic/dp/0742550524 Also useful for you since it kind of puts up Scholasticism vs Secular philosophy and where the Church broadly diverges (though always keep in mind that, even if Scholasticism is the most influential philosophical movement in the Church, Catholicism does not stamp it as the official philosophy of the Church.)