Aquinas's philosophical style in the Summa Theologiae has many virtues. Its systematic character makes it very difficult to dodge an issue, and the priority he gives to the opinions of his opponents locates his discussion in the real world of debate. But that was a thirteenth century world, and some of what is admirable in that context presents problems for modern students. His use of scriptural quotations, for instance, can baffle, bore or repel some of them. Admittedly the way Aquinas uses scripture constitutes a powerful cumulative demonstration of a non-fundamentalist approach by a major authority in the tradition, and that is a good reason for leaving them in. But that desideratum falls outside the scope of a treatise on the virtues. Again, some of the opponents' objections now lack the force they had then, even if they are sometimes illuminating on matters of importance, Augustine's thought for example. But that too is not central to the purpose of the work. So there is a strong case for a fairly drastic pruning, at least for some purposes.
There are perils here for a clumsy editor. Some of the material inviting excision is comparatively ephemeral but some is so integral to the work that its exclusion would leave us with a seriously distorted patchwork with few clues as to the overall world-view of the author. There is also a good deal of material that is contrary to contemporary culture and which timidity would suggest be excized. But Regan's editorial hand is deft. If mortal and venial sin, salvation, the value of consecrated virginity, the immorality of taking interest on a loan are unpalatable to the modern reader, he does not see that as any reason for excluding them. We find startling the prohibition of lying to the enemy in time of war, and the observation that it is permissible to resist an unjust death sentence "except perhaps, in order to avoid scandal, when resistance might risk a serious public disturbance" (86). This is just one example of the profoundly counter- cultural weighting of the community over the individual in the Natural Law tradition (This is very briefly but effectively flagged by Regan in his Introduction). Another example is in the treatment of "simple fornication" which Aquinas opposes chiefly on the grounds that you need two parents to raise a child. And he says that the particular circumstances of a case (plenty of money to provide for any child that might be produced) do not abrogate the law that casual sex is not right.
The extent of the condensation is considerable. The complete discussion of the cardinal virtues occupies six volumes of the sixty volume English Dominican Fathers edition of the 1960s. Cutting this by half to allow for that edition's inclusion of the Latin original leaves about 440 pages. Moreover Regan's 170 pages include the discussion of war, not included in those volumes. Surely he must have left out something of value? I searched for unfortunate omissions. Did he include the distinction between virtue and skill (in terms of intentional failure)? Yes. Does the thesis of the unity of the virtues survive? Yes again. Is the possibility of prudence in sinners included? Yes it is, and the search for omissions begins to seem a waste of time. Naturally some good material is left out, for example 2a2ae 52 concerning the gifts of the Holy Spirit. But that is definitely arcane at best for the typical modern reader. Overall, I think we can regard it as settled that the selection Regan offers is a fair and accurate account of the views of the great man.
It remains to consider the quality of the translation. Regan's reads well, but so have others that occasionally throw up bizarre supposed equivalences. A fine example of the genre (not Regan's doing) is the use of "transport" for "extasis" so that the question addressed at 1a2ae.28.3, "Utrum extasis sit effectus amoris" disconcertingly becomes "Is transport an effect of love?" That sort of thing plainly will not do. Regan offers few opportunities for criticism. A scan of initially suspect words and phrases yields the following:
At 2a2ae 50.1 (p. 15) the use of the phrase "polity (also called timocracy)" is an accurate translation of the Latin of the edition used, though not of all editions. The 1960s Dominican edition, which prefers the Piana edition to the Leonine used by Regan, leaves out "politeia" and thus puts timocracy in the foreground. Whatever the textual accuracy, the pedagogical purpose of the work would be better served by this translation, partly because "polity" is not a current English term for a particular sort of political system, and partly because the use of "timocracy" usefully relates this discussion to Plato's taxonomy in his Republic.
At 2a2ae 51.1 "euboulia" is translated as "good deliberation," which becomes "right deliberation" in the objection. Neither offers a clear insight into what is being discussed. I would think that "good counsel," or "being well-advised" would be at least as accurate, and superior from the point of view of securing understanding.
At 2a2ae 57.4 the translation of "ius dominativum" in terms of master and slave does not seem to be clearly justified by the original in either of the editions I consulted, and introduces an unnecessary complication, viz. the ancient and continuing institution of literal slavery.
Apart from these few cases, some terms resist translation that is both accurate and easily intelligible. "Astutia" at 2a 2ae 55.3 is a case in point. In our psychological/moral vocabulary it has a sense far removed from the question of sin, which is the context for the Summa discussion. "Craftiness," Regan's choice, is not a bad one, but it doesn't have the resonance its Latin equivalent evidently did in the 13th Century. I cannot think of a better alternative.
Overall, I would judge the translation to be a fine effort.
Finally, a couple of minor quibbles. If you use this edition to find passages identified by the conventional method of citing the Summa , you will have to work through a thicket of "ibids." This is unlikely to be a problem for the likely readership, but it was a slight annoyance for this reviewer. And I think the phrase "as it were" gets more employment than it deserves.
These criticisms are of little weight in the context of the many virtues of the book.