The Psalms in general exhibit the Lord Jesus and the godly (properly and specifically from among the Jews) in their mutual relations. He is identified with them, and they with Him; brought through darkness, trial, the contradiction of sinners, the often apparent and, in one sense and time, the real desertion of God, into security, peace, and blessing. This furnished the occasion, sometimes offered in the past circumstances of righteous Israelites and of David especially, for the Spirit of Christ in them to launch out into higher scenes and subjects, even the sufferings of Christ and the glories that should follow. At one and the same time there is an intermingling of the particular things and persons of the day in which these strains were written, and there is the anticipation of the latter-day tribulation, through which the Jewish remnant are destined to pass into the wide field of Millennial glory. Thus is drawn out the Holy Ghost's revealed expression of the feelings and experiences suitable to each and all.
The Psalms, consequently, do not bring the Church, as a distinctive body, to light, if we except some indirect allusions which we understand, now that the mystery, hidden from ages and generations, is made manifest. In this respect they resemble the Old Testament prophecies. But there is also this striking difference, that while the prophets, for the most part, narrate the sufferings and triumphs of Christ as the head of Israel and the Gentiles as predicted facts, the Psalms lay bare the inmost privacy of His and their hearts as brought into exercise by these circumstances. Hence, while the prophecies chiefly reveal the feelings of God about Christ and His servants, the Psalms chiefly reveal the feelings of Christ and His servants about God. There are no doubt large and frequent exceptions, but this is, I think, a generally characteristic difference between these portions of the Bible.
But again, the Psalms are, as is familiar to the reader of the Hebrew Bible, divided into five books. Nor are these divisions arbitrary. Various marks are impressed on them by God, which show that this is no rabbinical fancy. Thus, even externally, it is plain that at the end of Psalm 41; 72, and 89, we have "Amen and Amen"; next, at the end of Psalm 106, "Amen. Praise ye the LORD," and thence, to the end of all, another class. These, with other common features in the verses where they occur, define the various books.
But the subjects, internally, differ thus:
Book 1 (Psalm 1-41) embraces Messiah's sympathy with the godly remnant in "the beginning of sorrows." They are not yet driven out, but are outwardly associated with the mass of the people, even in worship. Hence the name of Jehovah is regularly there.
Book 2 (Psalm 42-72) views the remnant as no longer in the land, but the object of hostility, not only of Gentiles, but of Jews united with them. The abomination of desolation is set up, and the tribulation is come. Accordingly, God is spoken of as such, save where hope is expressed.
Book 3 (Psalm 73-89) is occupied, not with Judah only, but with Israel, and also with a wider range of foreign enemies. It is founded on God's ways with the whole people.
Book 4 (Psalm 90-106) celebrates the bringing Christ again into the world, and hence is the book of Millennial blessedness.
Book 5 (Psalm 107-150) reviews all, opens out the principles of God's dealings, and of relationships with Him, and gives the grand result of all the discipline, and the subsequent blessing of God. Its thanksgivings at the end are thus the moral answer to the groanings of the Spirit in book 1.