A few months ago J. Budziszewski (University of Texas at Austin) published a line by line commentary on Summa theologiae, I-II, qq. 90-97 with Cambridge University Press entitled (aptly enough) Commentary on Thomas Aquinas's Treatise on Law. I am not aware of another such commentary in English. Here is CUP's description. 

Natural moral law stands at the center of Western ethics and jurisprudence and plays a leading role in interreligious dialogue. Although the greatest source of the classical natural law tradition is Thomas Aquinas' Treatise on Law, the Treatise is notoriously difficult, especially for nonspecialists. J. Budziszewski has made this formidable work luminous. This book - the first classically styled, line by line commentary on the Treatise in centuries - reaches out to philosophers, theologians, social scientists, students, and general readers alike. Budziszewski shows how the Treatise facilitates a dialogue between author and reader. Explaining and expanding upon the text in light of modern philosophical developments, he expounds this work of the great thinker not by diminishing his reasoning, but by amplifying it.

You can find additional material related to the book at  Budziszewski's website The Underground Thomist. Included in the material is a PDF file of a Companion to the Commentary, which features a line by line treatment of selections from qq. 98-108 along with further discussion of topics from various articles across the Treatise. The Companion itself is 239 pages! Budziszewski says that the material in the Companion was not added to the original book because then the entire text would have come out to 800 pages and "a book shouldn't be a concrete block." Well, I would have still bought it. Some of the best books I own are concrete blocks.

People familiar with Budziszewski's work know that he has been writing (and teaching) about natural law, especially Aquinas's version of it, for many years. So, he was well prepared to write this book. It is surprising that he did not write it sooner! At any rate, I think it will prove a valuable resource.