The Structure of the Psalms
God ordered Ezra, the scholar and priest, “Collect and compile the Psalms of the Spirit.” The scrolls were edited when God took the Israelites out of Babylon, the house of the captivity, as God promised Abraham and David.
There are many authors of the Psalms including David, Solomon, Moses, and Asaph. The Psalms come in different styles from different eras, situations, and contexts. It is recorded that Solomon wrote as many as 1005 songs (1Kg 4: 32).
Ezra collected the Psalms and did not arrange them in chronological order or occasion. He also did not arrange them by author. He did not sort by genre such as psalms of praise, psalms of wisdom, or psalms of lament either.
Instead, he decided to arrange the Psalms into books representing the covenant given to the people by God and books that responded with thanks and praise from the people to God, an expression of the love of the covenant between God and his people.
The editor chose the theme "The Lord is the Rock" from Deut. 32, Moses' song, in the first book of covenants given to the people by God. The Lord of the covenant is the Rock King. In the last words of Moses in the following chapter (Deut. 33) it is sung for the first time, "Blessed are those who ..." He chose a psalm which reminds us of Moses' disciple Joshua as Psalm 1. The song "Lord is the Rock" is also reproduced in the song of David (2 Samuel 22). He places it in the psalter as Psalm 18.
Then the Lord leads his own sheep as a King and holy one, making them his own people. He leads the people to the Promised Land. The editor will arranges this psalm as Psalm 90, Moses' song "Lord, thou hast been our dwelling place ..." as a response of praise from the people to God.
Time passed, then it was the era of the king of David. The Lord responded to David's prayers and promised him son of David would sit on the throne. And the king promises to build a house and establish a kingdom. As promised, Solomon then sits on the throne of his father David. Hee builds the temple of God. Psalm 72 comes at the end of Book 2, singing that Solomon will come to the throne. is By the end of Book 3, Psalm 89 shows an established covenant with the Kingdom of David.
These sections can be titled "The Lord is a Rock" for Book 1, "The Son becomes a King" for Book 2, "House of God" for Book 3, and "God dwells among His People" for Book 4.
This time, in commemoration of the fact that God brought us out of the country of Babylon as he swore to Abraham and David, the people will respond to the blessings God has kept the covenant.
As David ordered the Levites in the tent, "O give thanks unto the Lord for He is good for His mercy endureth forever. Hallelujah!”, as marked by this refrain of praise, Book 5 is divided into four subsections: Ps.107-108 / Ps.119 / Ps.120-134 / Ps.135-150.
The second subsection of "for He is good" is Psalm 119 which is a song on the Word of God. And in the third subsection, the psalms, known as the Psalms of Ascents (Ps.120-Ps.134) give thanks for guidance, singing "for His mercy endureth forever"..
Book 5 is also consistent with the flow of the feasts, commemorative festivals when the people go up to Jerusalem and give thanks and praise. The first subsection matches the January Passover. The second subsection corresponds with the Seven Weeks of March. The third subsection with the 40 years in wilderness (4 months). The fourth subsection with the feast of Tabernacle in July.
People are brought out at the Passover, and the Words are given as treasures at Mount Sinai (Psalm 119). And the people are led to the city of Jerusalem (Ascent). Finally, they enter the new city and are pleased with the new creation (Tabernacle).
Book 1 to Book 4 represent Love from God. 1. The Lord is a Rock, 2. The Son becomes a King, 3. House of God, 4. God dwells among His People.
Book 5 is the response to God from the people. "O give thanks unto the Lord for He is good for His mercy endureth forever. Hallelujah!"
Just like the book of Psalms has its own sequence, each of the five books contain their own covenant sub-structures.