The entire Book of Concord seems daunting, but it is a book of Confessions, not War and Peace.
The Augsburg Confession is a concise masterpiece, except the last part needs more of a plot for our days. The short concise statements are worthy of memorization. Melanchthon was the primary author.
Article IV: Of Justification.
1] Also they teach that men cannot be justified before God by their own strength, merits, or works, but are freely justified for 2] Christ's sake, through faith, when they believe that they are received into favor, and that their sins are forgiven for Christ's sake, who, by His death, has made satisfaction for our sins. 3] This faith God imputes for righteousness in His sight. Rom. 3 and 4.
The Apology is Melanchthon's defense of the Augsburg Confession. If you want more detail about justification, read the Apology's section on that topic.
But that faith which justifies is not merely a knowledge of history, [not merely this, that I know the stories of Christ's birth, suffering, etc. (that even the devils know,)] but it is to assent to the promise of God, in which, for Christ's sake, the remission of sins and justification are freely offered. [It is the certainty or the certain trust in the heart, when, with my whole heart, I regard the promises of God as certain and true, through which there are offered me, without my merit, the forgiveness of sins, grace, and all salvation, through Christ the Mediator.] And that no one may suppose that it is mere knowledge, we will add further: it is to wish and to receive the offered promise of the remission of sins and of justification. [Faith is that my whole heart takes to itself this treasure. It is not my doing, not my presenting or giving, not my work or preparation, but that a heart comforts itself, and is perfectly confident with respect to this, namely, that God makes a present and gift to us, and not we to Him, that He sheds upon us every treasure of grace in Christ.]
49] And the difference between this faith and the righteousness of the Law can be easily discerned. Faith is the latreiva [divine service], which receives the benefits offered by God; the righteousness of the Law is the latreiva [divine service] which offers to God our merits. By faith God wishes to be worshiped in this way, that we receive from Him those things which He promises and offers.
The Large Catechism is valuable because it consists of Luther's sermons edited to cover in detail the content of the Small Catechism, which people take for granted as they chant it. Many grave doctrinal errors are addressed by the Large Catechism. If someone wants a set of sermons on the Catechism - here they are. If a pastor needs ideas for a sermon - read the LC.
99] Likewise those fastidious spirits are to be reproved who, when they have heard a sermon or two, find it tedious and dull, thinking that they know all that well enough, and need no more instruction. For just that is the sin which has been hitherto reckoned among mortal sins, and is called ajkhdia, i.e., torpor or satiety, a malignant, dangerous plague with which the devil bewitches and deceives the hearts of many, that he may surprise us and secretly withdraw God's Word from us.
100] For let me tell you this, even though you know it perfectly and be already master in all things, still you are daily in the dominion of the devil, who ceases neither day nor night to steal unawares upon you, to kindle in your heart unbelief and wicked thoughts against the foregoing and all the commandments. Therefore you must always have God's Word in your heart, upon your lips, and in your ears. But where the heart is idle, and the Word does not sound, he breaks in and has done the damage before we are aware. 101] On the other hand, such is the efficacy of the Word, whenever it is seriously contemplated, heard, and used, that it is bound never to be without fruit, but always awakens new understanding, pleasure, and devoutness, and produces a pure heart and pure thoughts. For these words are not inoperative or dead, but creative, living words. 102] And even though no other interest or necessity impel us, yet this ought to urge every one thereunto, because thereby the devil is put to Right and driven away, and, besides, this commandment is fulfilled, and [this exercise in the Word] is more pleasing to God than any work of hypocrisy, however brilliant.
The Formula of Concord is the masterpiece of Chemnitz' career. All his scholarship went into that Confession, and he had the help of other great theologians, like Chytraeus.
If someone wants to study Law and Gospel, the Formula is the place to start. There are many other places in the BOC where the topic comes up. The Concordance to the Book of Concord is handy for finding those passages. That is how I found the efficacy passages in the BOC for Thy Strong Word.
V. Law and Gospel
1] As the distinction between the Law and the Gospel is a special brilliant light, which serves to the end that God's Word may be rightly divided, and the Scriptures of the holy prophets and apostles may be properly explained and understood, we must guard it with especial care, in order that these two doctrines may not be mingled with one another, or a law be made out of the Gospel, whereby the merit of Christ is obscured and troubled consciences are robbed of their comfort, which they otherwise have in the holy Gospel when it is preached genuinely and in its purity, and by which they can support themselves in their most grievous trials against the terrors of the Law.
Another issue, often used but abused, is adiaphora, employed to excuse aping the Enthused -
10] We believe, teach, and confess also that at the time of confession [when a confession of the heavenly truth is required], when the enemies of God's Word desire to suppress the pure doctrine of the holy Gospel, the entire congregation of God, yea, every Christian, but especially the ministers of the Word, as the leaders of the congregation of God [as those whom God has appointed to rule His Church], are bound by God's Word to confess freely and openly the [godly] doctrine, and what belongs to the whole of [pure] religion, not only in words, but also in works and with deeds; and that then, in this case, even in such [things truly and of themselves] adiaphora, they must not yield to the adversaries, or permit these [adiaphora] to be forced upon them by their enemies, whether by violence or cunning, to the detriment of the true worship of God and the introduction and sanction of idolatry. 11] For it is written, Gal. 5:1: Stand fast, therefore, in the liberty wherewith Christ hath made us free, and be not again entangled in the yoke of bondage. Also Gal. 2:4f : And that because of false brethren unawares brought in, who came in privily to spy out our liberty which we have in Christ Jesus, that they might bring us into bondage; to whom we gave place by subjection, no, not for an hour, that the truth of the Gospel might continue with you. 12] [Now it is manifest that in that place Paul speaks concerning circumcision, which at that time had become an adiaphoron (1 Cor. 7:18f.), and which at other occasions was observed by Paul (however, with Christian and spiritual freedom, Acts 16:3). But when the false apostles urged circumcision for establishing their false doctrine, (that the works of the Law were necessary for righteousness and salvation,) and misused it for confirming their error in the minds of men, Paul says that he would not yield even for an hour, in order that the truth of the Gospel might continue unimpaired.]
Another Luther gem - the Smalcald Articles. There we are inoculated against Enthusiasm. The Enthusiasts in WELS, ELCA, LCMS, and the Little Sect ran rampant today, glorying in their shame, proving that no one reads this confession, cares about it, defends it, or teaches it. Remember that fact the next time the synod has its lupine paws out for offerings - they are collecting money to broadcast the complete denial of Biblical doctrine. They are Walking Together - with Satan - hooves clicking like metronomes as they march down the broad path.
3] And in those things which concern the spoken, outward Word, we must firmly hold that God grants His Spirit or grace to no one, except through or with the preceding outward Word, in order that we may [thus] be protected against the enthusiasts, i.e., spirits who boast that they have the Spirit without and before the Word, and accordingly judge Scripture or the spoken Word, and explain and stretch it at their pleasure, as Muenzer did, and many still do at the present day, who wish to be acute judges between the Spirit and the letter, and yet know not what they say or declare. 4] For [indeed] the Papacy also is nothing but sheer enthusiasm, by which the Pope boasts that all rights exist in the shrine of his heart, and whatever he decides and commands with [in] his church is spirit and right, even though it is above and contrary to Scripture and the spoken Word.
5] All this is the old devil and old serpent, who also converted Adam and Eve into enthusiasts, and led them from the outward Word of God to spiritualizing and self-conceit, and nevertheless he accomplished this through other outward words. 6] Just as also our enthusiasts [at the present day] condemn the outward Word, and nevertheless they themselves are not silent, but they fill the world with their pratings and writings, as though, indeed, the Spirit could not come through the writings and spoken word of the apostles, but [first] through their writings and words he must come. Why [then] do not they also omit their own sermons and writings, until the Spirit Himself come to men, without their writings and before them, as they boast that He has come into them without the preaching of the Scriptures? But of these matters there is not time now to dispute at greater length; we have elsewhere sufficiently urged this subject.