An excellent treatise on the law and Gospel, justification, sanctification. The first few chapters provide a helpful discussion of the Mosaic covenant and third use of the law for believers.
"Were we, indeed, to perform but a single act of obedience more than we owed to God, we would thereby merit some recompense from Him. But this is impossible for us ever to do. It is not the obedience even of a true believer that merits the blessings of salvation for him, but only the meritorious righteousness of Christ imputed to him" (pg. 80-81).
Colquhoun is also very helpful in distinguishing sanctification from justification and the proper role of good works in the life of believers. Good works are Gospel driven, sanctification flows forth from justification. This is very helpful for a proper understanding of both sanctification and assurance for believers.
"To pretend to sanctification, and then to rely on it for justification, is to derive the fountain from the stream, the cause from the effect, and so to invert the order of the blessings from salvation. It is necessary that our sins are forgiven, and our persons accepted as righteous in the sight of God, in order to our being capable of yielding the least degree of acceptable obedience to Him" (pg. 309).