https://www.amazon.ca/Aquinas-Beginners-Guide-Edward-Feser/dp/1851686908
Although the title says it is a beginner's guide, I would use this more to deepen my understanding of Aquinas' basic philosophy. As an introduction, it delves deeper into certain issues than beginners may be comfortable with. Especially if they are unfamiliar with Aristotlelian/Thomistic metaphysics. 
I would recommend starting with Feser's "The Last Superstition," where he goes into basic Platonic and especially Aristotelian metaphysics. These set the groundwork for better understanding Aquinas' thought, and much of what is in this book. These considerations aside, this book was a great tool for further understanding Aquinas' thought, especially in light of current modern misunderstandings and objections. Must read.