Only 124 pages, yet Brown covers a lot of ground, surveying that period of history with concise historical summaries, quotes, and insights. He looks at the covenant theology of Calvin, Ursinus, Olevanius, Rollock, Perkins, Polanus, Wollebius, Ames, Sibbes, Ball, Bolton, Strong, Rutherford, Dickson, Calamy, Cocceius, Patrick Gillespie, Owen, Turretin, Witsius, and of course Petto. The question in focus throughout is, "how did the Mosaic Covenant relate to the Covenant of Works and the Covenant of Grace?" Brown documents what were a variety of approaches to that question among these many theologians. Crucial to these Reformed theologians, and especially Petto, was understanding the Mosaic covenant in such a way as to not impinge upon the unconditional promise of salvation in, and the continuity of, the Covenant of Grace throughout Scripture. And very much related to that was their view that the Mosaic covenant was not a re-inauguration of the Covenant of Works for one's salvation.
Here are two shorts excerpt taken from the book:
"The better Covenant and that at Sinai, are contradistinguished, and so must be two distinct Covenants, else the opposition were groundless, Jer. 31.31, 32 -- I will make a New Covenant -- Not according to the Covenant I made with their Fathers -- i.e. not according to the Sinai Covenant; for that was it which was made when they were brought out of the Land of Egypt. He doth not say, I will set up a new administration of my Covenant (though that had been true) but a New Covenant; there is a plain opposition between Covenant and Covenant, and therefore the New and that at Sinai must be two distinct, and not one and the same in two different forms; and the rather, because this New Covenant is not opposed to the Covenant with Abraham, and to that with David, but only to that with Moses and Israel at Mount Sinai..." (Petto, p. 101)
... He [Petto] refused to call faith, repentance, or obedience "conditions," either antecedent or subsequent ones. He acknowledged that if there was a condition for believers in the new covenant, it would seem to be faith, yet that cannot be the case. A condition "properly taken," he argued, earns the right to the benefit promised. This, said Petto, cannot apply to faith, because faith receives a benefit; it does not earn a right to it. While recognizing that the New Testament often uses conditional language to speak of the necessity of faith, repentance, and obedience, he stressed that these are gifts earned by Christ's obedience and bestowed upon believers by the inward working of the Holy Spirit. "In the very Covenant it self, it is promised that he will write his Laws on their hearts, Heb. 8.10. and that inplyeth Faith, Repentance, and every gracious frame." (p. 113)[The quotes that Brown uses here are from Petto's Difference between the Old and New Covenant]