IT is the consensus of modern scholarly opinion that the Book of Psalms as we now have it is the combination of many previous smaller collections. (See Introduction, page 6 ff.) It is not necessary to go into a detailed argument in support of this opinion, but a few of the more obvious arguments may be mentioned. Book Two ends with the statement, "The prayers of David the son of Jesse are ended." ( Psalm 70:20.) Yet, in spite of that statement, there is a psalm ascribed to David in Book Three, two psalms in Book Four, and fifteen psalms in Book Five. Evidently the sentence: "The prayers of David are ended," was appended to the end of Book Two at a time when that was a separate and complete collection. It also is to be noted that whereas Book One uses, almost exclusively, the name JHWH for God, Books Two and Three use (with few exceptions) the name Elohim. This clearly points to the fact that Books Two and Three were part of one collection whose editor preferred to use Elohim as the name of God.
Book Two itself must have been composed of two earlier collections. Psalms 42 to 49 have the heading, "of the sons of Korah" (a Levitical family), and Psalms 51 to 70 (with the exception of 66 and 67) have the heading, "Of David." It is interesting to note that in this collection of "David" psalms found in Book Two, there are two psalms which are duplicates of the David psalms found in the collection which constitute Book One. Thus, Psalm 53 is virtually the exact duplicate of Psalm 14 in the earlier collection, and Psalm 70 is identical with Psalm 40:14-18.
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Publication information: Book title: The Book of Psalms:A Commentary. Contributors: Solomon B. Freehof - Author. Publisher: Union of American Hebrew Congregations. Place of publication: Cincinnati. Publication year: 1938. Page number: 109.