*Minor differences ignored. Grouped by changes, with first version listed as example.
The perfect law of liberty. After having spoken of empty speculation, he comes now to that penetrating intuition which transforms us to the image of God. And as he had to do with the Jews, he takes the word law, familiarly known to them, as including the whole truth of God. But why he calls it a perfect law, and a law of liberty, interpreters have not been able to understand; for they have not perceived that there is here a contrast, which may be gathered from other passages of Scripture. As long as the law is preached by the external voice of man, and not inscribed by the finger and Spirit of God on the heart, it is but a dead letter, and as it were a lifeless thing. It is, then, no wonder that the law is deemed imperfect, and that it is the law of bondage; for as Paul teaches in Galatians 4:24, separated from Christ, it generates to condemn and as the same shews to us in Romans 8:13, it can do nothing but fill us with diffidence and fear. But the Spirit of regeneration, who inscribes it on our inward parts, brings also the grace of adoption. It is, then, the same as though James had said, "The teaching of the law, let it no longer lead you to bondage, but, on the contrary, bring you to liberty; let it no longer be only a schoolmaster, but bring you to perfection: it ought to be received by you with sincere affection, so that you may lead a godly and a holy life." Moreover, since it is a blessing of the Old Testament that the law of God should reform us, as it appears from Jeremiah 31:33, and other passages, it follows that it cannot be obtained until we come to Christ. And, doubtless, he alone is the end and perfection of the law; and James adds liberty, as an inseparable associate, because the Spirit of Christ never regenerates but that he becomes also a witness and an earnest of our divine adoption, so as to free our hearts from fear and trembling. And continueth. This is firmly to persevere in the knowledge of God; and when he adds, this man shall be blessed in his deed, or work, he means that blessedness is to be found in doing, not in cold hearing. 
1 - It may be rendered thus, -- "The same shall be blessed in (or by) the doing of it," that is, the work. The very doing of the law of liberty, of what the gospel prescribes, makes a man blessed or happy.
But whoso looketh - (παρακύψας parakupsas). This word means, to stoop down near by anything; to bend forward near, so as to look at anything more closely. See the word explained in the notes at 1-Peter 1:12. The idea here is that of a close and attentive observation. The object is not to contrast the manner of looking in the glass, and in the law of liberty, implying that the former was a "careless beholding," and the latter an attentive and careful looking, as Doddridge, Rosenmuller, Bloomfield, and others suppose; for the word used in the former case (κατενόησε katanoēse) implies intense or accurate observation, as really as the word used here; but the object is to show that if a man would attentively look into, and continue in the law of liberty, and not do as one who went away and forgot how he looked, he would be blessed. The emphasis is not in the manner of looking, it is on the duty of continuing or persevering in the observance of the law.
The perfect law of liberty - Referring to the law of God or his will, however made known, as the correct standard of conduct. It is called the perfect law, as being wholly free from all defects; being just such as a law ought to be. Compare Psalm 19:7. It is called the law of liberty, or freedom because it is a law producing freedom from the servitude of sinful passions and lusts. Compare Psalm 119:45; Notes, Romans 6:16-18.
And continueth therein - He must not merely look at the law, or see what he is by comparing himself with its requirements, but he must yield steady obedience to it. See the notes at John 14:21.
This man shall be blessed in his deed - Margin, doing. The meaning is, that he shall be blessed in the very act of keeping the law. It will produce peace of conscience; it will impart happiness of a high order to his mind; it will exert a good influence over his whole soul. Psalm 19:11. "In keeping of them there is great reward."
But whoso looketh into the perfect law - The word παρακυψας, which we translate looketh into, is very emphatic, and signifies that deep and attentive consideration given to a thing or subject which a man cannot bring up to his eyes, and therefore must bend his back and neck, stooping down, that he may see it to the greater advantage. The law of liberty must mean the Gospel; it is a law, for it imposes obligations from God, and prescribes a rule of life; and it punishes transgressors, and rewards the obedient. It is, nevertheless, a law that gives liberty from the guilt, power, dominion, and influence of sin; and it is perfect, providing a fullness of salvation for the soul: and it may be called perfect here, in opposition to the law, which was a system of types and representations of which the Gospel is the sum and substance. Some think that the word τελειον, perfect, is added here to signify that the whole of the Gospel must be considered and received, not a part; all its threatenings with its promises, all its precepts with its privileges.
And continueth - Παραμεινας· Takes time to see and examine the state of his soul, the grace of his God, the extent of his duty, and the height of the promised glory. The metaphor here is taken from those females who spend much time at their glass, in order that they may decorate themselves to the greatest advantage, and not leave one hair, or the smallest ornament, out of its place.
He being not a forgetful hearer - This seems to be a reference to Deuteronomy 4:9 : "Only take heed to thyself, and keep thy soul diligently, lest thou forget the things which thine eyes have seen, and lest they depart from thy heart all the days of thy life." He who studies and forgets is like to a woman who brings forth children, and immediately buries them. Aboth R. Nathan, cap. 23.
Shall be blessed in his deed - In Pirkey Aboth, cap. Deuteronomy 4:14, it is said: "There are four kinds of men who visit the synagogues,
1. He who enters but does not work;
2. He who works but does not enter.
3. He who enters and works.
4. He who neither enters nor works.
The first two are indifferent characters; the third is the righteous man; the fourth is wholly evil."
As the path of duty is the way of safety, so it is the way of happiness; he who obeys God from a loving heart and pure conscience, will infallibly find continual blessedness.
But whoso looketh into the perfect law of liberty, and continueth [therein], he being not a forgetful hearer, but a doer of the work, this man shall be blessed in his (x) deed.
(x) Behaviour: for works show faith.
But whoso looketh into the perfect law of liberty,.... By which is meant, not the moral law, but the Gospel; for only of that is the apostle speaking in the context: this is no other than the word of truth, with which God begets men of his own will; and is the ingrafted word which is able to save, and of which men should be doers, as well as hearers, James 1:18, and this is compared to a glass by the Apostle Paul, 2-Corinthians 3:18, and the word here used for looking into it is the same word the Apostle Peter uses of the angels, who desired to look into the mysteries of the Gospel, 1-Peter 1:12 all which serve to strengthen this sense; now the Gospel is called a law; not that it is a law, strictly speaking, consisting precepts, and established and enforced by sanctions penalties; for it is a declaration of righteousness and salvation by Christ; a publication of peace and pardon by him; and a free promise of eternal life, through him; but as it is an instruction, or doctrine: the law with the Jews is called because it is teaching and instructive; and everything that is so is by them called by this name: hence we find the doctrine of the Messiah, which is no other than the Gospel, is in the Old Testament called the law of the Lord, and his law, Isaiah 2:2 and in the New Testament it is called the law, or doctrine of faith, Romans 3:27 and this doctrine is perfect, as in Psalm 19:7, it being a perfect plan of truths, containing in it all truth, as it is in Jesus, even all the treasures of wisdom and knowledge; and because it is a revelation of things perfect; of the perfect righteousness of Christ, and of perfect justification by it, and of free and full pardon of sins through him, and of complete salvation by him; and because it directs to Christ, in whom perfection is: and it is a law or doctrine of liberty; , "that which is if liberty"; which has liberty for its subject, which treats of it, even of the liberty wherewith Christ makes his people free: the Gospel proclaims this liberty to captive souls; and is the word of truth, which makes them free, or is the means of freeing them from the slavery of sin, from the captivity of Satan, and from the bondage of the law; and is what gives souls freedom and boldness at the throne of grace; and is that which leads them into the liberty of grace here, and gives them a view and hope of the glorious liberty of the children of God hereafter. This doctrine is as a glass to look into; in which is beheld the glory of Christ's person and office, and grace; and though by the law is the knowledge of sin, yet a man never so fully and clearly discovers the sin that dwells in him, and the swarms of corruption which are in his heart, as when the light of the glorious Gospel shines into him, and when in it he beholds the beauty and glory of Jesus Christ; see Isaiah 6:5 and looking into this glass, or into this doctrine, is by faith, and with the eyes of the understanding, opened and enlightened by the Spirit of God; and the word here used signifies a looking wistly and intently, with great care and thought, and not in a slight and superficial manner; and such a looking is designed, as is attended with effect; such an one as transforms into the same image that is beheld, from glory to glory; and happy is the man that so looks into it.
And continueth therein; is not moved away from the hope of the Gospel, nor carried about with divers and strange doctrines; but is established in the faith, stands fast in it, and abides by it; or continues looking into this glass, and to Christ, the author and finisher of faith, who is beheld in it; and keeps his eye upon it, and the object held forth in it; and constantly attends the ministration of it:
he being not a forgetful hearer; but takes heed to the things he hears and sees, lest he should let them slip; and being conscious of the weakness of his memory, implores the divine Spirit to be his remembrancer, and bring to his mind, with fresh power and light, what he has heard:
but a doer of the work; of the work of faith, and labour of love, and patience of hope, and of every work and ordinance the Gospel ministry points unto; doing and being subject to all in faith, from a principle of love, and with a view to the glory of God and Christ.
This man shall be blessed in his deed; or "doing", and while he is doing; not that he is blessed for what he does, but "in" what he does; see Psalm 19:11 he having, in hearing the word, and looking into it, and in submitting to every ordinance of the Gospel, the presence of God, the discoveries of his love, communion with Christ, and communication of grace from him by the Spirit; so that Wisdom's ways become ways of pleasantness, and all her paths are peace; see Psalm 65:4, moreover, in all such a man does, he is prosperous and successful; in all he does he prospers: and so he is blessed in his deed, by God, whose blessing makes rich, both in spirituals and temporals: there seems to be an allusion to the blessed man in Psalm 1:1.
looketh into--literally, "stoopeth down to take a close look into." Peers into: stronger than "beholdeth," or "contemplated," James 1:24. A blessed curiosity if it be efficacious in bearing fruit [BENGEL].
perfect law of liberty--the Gospel rule of life, perfect and perfecting (as shown in the Sermon on the Mount, Matthew 5:48), and making us truly walk at liberty (Psalm 119:32, Church of England Prayer Book Version). Christians are to aim at a higher standard of holiness than was generally understood under the law. The principle of love takes the place of the letter of the law, so that by the Spirit they are free from the yoke of sin, and free to obey by spontaneous instinct (James 2:8, James 2:10, James 2:12; John 8:31-36; John 15:14-15; compare 1-Corinthians 7:22; Galatians 5:1, Galatians 5:13; 1-Peter 2:16). The law is thus not made void, but fulfilled.
continueth therein--contrasted with "goeth his way," James 1:24, continues both looking into the mirror of God's word, and doing its precepts.
doer of the work--rather, "a doer of work" [ALFORD], an actual worker.
blessed in his deed--rather, "in his doing"; in the very doing there is blessedness (Psalm 19:11).
But he that looketh diligently - Not with a transient glance, but bending down, fixing his eyes, and searching all to the bottom. Into the perfect law - Of love as established by faith. St. James here guards us against misunderstanding what St. Paul says concerning the "yoke and bondage of the law." He who keeps the law of love is free, John 8:31, &c. He that does not, is not free, but a slave to sin, and a criminal before God, James 2:10. And continueth therein - Not like him who forgot it, and went away. This man - There is a peculiar force in the repetition of the word. Shall be happy - Not barely in hearing, but doing the will of God.
*More commentary available at chapter level.