Question: "Why is the book of Psalms divided into five books?"
Answer: The book of Psalms is the longest book of the Bible, with 150 “chapters”—more properly called “psalms” or “songs.” Psalms is divided into five books:
Book 1: Psalms 1—41
Book 2: Psalms 42—72
Book 3: Psalms 73—89
Book 4: Psalms 90—106
Book 5: Psalms 107—150
It is uncertain why Psalms is divided into five books. Some sources, including Jewish Midrash traditions, suggest the five-fold division is based on the five books of the Torah (Genesis to Deuteronomy). The division of the Psalms is not based on authorship or chronology, as several authors composed Psalms, and their individual songs are mixed throughout the various collections.
David is listed as the author of 73 psalms, Asaph of 12, and the sons of Korah of 11. Other psalms were written by Solomon, Heman the Ezrahite, Ethan the Ezrahite, and Moses (Psalm 90). The earliest extant copy of Psalms is from the Dead Sea Scrolls from about the first century AD. That copy shows that the division into five books extends to at least that time and certainly earlier.
It is most likely that Ezra and/or other Jewish religious leaders compiled the Psalms into their existing order during Ezra’s lifetime in the fourth century BC. Interestingly, the Psalms was one of the most popular writings among the Dead Sea Scrolls, with thirty scrolls of all or parts of the book included. Overall, Psalms is the book of the Old Testament with the most Hebrew manuscripts available for research, indicating its enduring popularity among both Jews and Christians.
Each of these five books or sections of Psalms ends with a doxology or a song of praise. The final verse of each concluding psalm includes either “Praise the Lord!” or “Amen.” For example, the final verse of Psalm 41 ends this way: “Praise be to the Lord, the God of Israel, / from everlasting to everlasting. / Amen and Amen.” Psalm 150, the final psalm, serves as the fitting final doxology, concluding with the words, “Let everything that has breath praise the Lord. / Praise the Lord.”