Thomas Aquinas, Treatise on Law, Summa Theologica I-II, 90 – 97.
Available in edited form, with explanatory notes: Peter Kreeft, A Summa of the Summa (Ignatius 1990). The Summa is also excerpted and discussed in Jeffrey A. Brauch, Is Higher Law Common Law? Readings on The Influence of Christian Thought in Anglo-American Law (Fred B. Rothman & Co. 1999).
The Summa is one of the most important works of western philosophy and Christian theology. The Encyclopedia Britannica notes that Thomas “has done more than any other writer save Augustine to fashion the theological language of the Western Church.” In the Treatise on Law, Thomas addresses the nature of God’s law and its relation to man, the role of the law, and divine law. There is no theory of natural law that is not indebted to Thomas Aquinas.
While relatively short, the Treatise on Law is not a quick read. It is written in the scholastic style. He begins with the proposition stated, then addresses contrary arguments, makes an evidentiary statement, and then presents his argument in the form of a refutation of the contrary arguments. I recommend reading the Treatise on Law with the help of Kreeft’s notes and explanations.
There are other editions, including a 1998 paperback from Regnery Publishing (Gateway Editions), with a fine, but short introduction by Professor Ralph McInerny. In addition, there are many good works that discuss Thomistic legal and political theory. See G.K. Chesterton, St. Thomas Aquinas (1933), John M. Finnis, Aquinas: Moral, Political and Legal Theory (1998) (Foundations of Political Thought Series), A.P. d’Entreves, The Medieval Contribution to Political Thought (Humanities Press 1959), and Anto-Hermann Chroust, The Philosophy of Law of St. Thomas Aquinas: His Fundamental Ideas and Some of His Historical Precursers, 19 Am. J. Juris. 1(1973).