I find that I need to be oriented to an author before I can really make intelligent use of primary texts. It will help you to understand Aquinas if you get a sense of when he lived, what people were doing, what he was trying to do, what he did and did not have access to, and what in general he accomplished. Here are seven guides.
Chesterton, G. K. St. Thomas Aquinas: ‘The Dumb Ox’. Someone somewhere says that it is remarkable how right Chesterton is on Aquinas while being so inaccurate. Chesterton is good for making you interested in Aquinas, rather than being someone you consult for getting details straight or understanding something difficult.
Pieper, Josef. Guide to Thomas Aquinas. I’m not even sure that I’ve read this, but I’ve read enough of Pieper to know he will be good on Aquinas. I read The Silence of St. Thomas most recently. If it is in the Guide that Pieper makes the wondrous suggestion that in the end Aquinas turned from Aristotle to Plato, then I’ve read it. Pieper is concise, easy to read, clear, and the most platonic of all the Thomists I’ve encountered.
Copleston, F. C. Aquinas: An Introduction to the Life and Work of the Great Medieval Thinker. Copleston’s is not the most recent or the most thorough, but it is reliable and shorter than those who are more detailed and more thorough. You may wish to start with one who is more simple. People still swear by his multivolume history of philosophy. This is a very decent book you might easily find used.
Davies, Brian. The Thought of Thomas Aquinas. If you were only to get one, this one may be it. If your criterion is detail, scope, and not the loveliness of the font or the niceness of the binding, if, in short, substance over style is your thing, Davies is the man. Aquinas was a thinker, and this gives you access to his thinking.
Kenny, Anthony John Patrick. Aquinas. Kenny is the odd atheist who appreciates Aquinas, to a point. It is quite short, it is not altogether always helpful, but it exists.
Shields, Christopher John, and Robert Pasnau. The Philosophy of Aquinas. Second Edition. This can function as a reference work. Need to look something up that is accurate and recent? This is topical.
Turner, Denys. Thomas Aquinas: A Portrait. If I only got one of these books and I cared about having an enjoyable book, this would be the one. It is not as detailed as Davies, it is not as consultable as Shields & Pasnau, but it is elegant in font and binding, reads like a novel, is amazingly substantial, and explains why it distorts. His aim is a caricature, but it is a useful, sympathetic, illuminating caricature. I’d say it was the best book on Aquinas I’ve read.