I am sometimes asked what book or books I would recommend as an introduction to the thought of St. Thomas Aquinas for the intelligent non-specialist.
Here is my advice. You should first read Ralph McInerny's entry on St. Thomas in the Stanford Encyclopedia of Philosophy. Then you can move on to these three books, which are some of my favorites:
Ralph McInerny, A First Glance at Thomas Aquinas: A Guide for Peeping Thomists (Notre Dame, IN: University of Notre Dame Press, 1990)
Brian Davies, O.P., The Thought of Thomas Aquinas (Oxford: Clarendon, 1992)
Edward Feser, Aquinas: A Beginner's Guide (Oxford: Oneworld Publications, 2009)