The Scofield Bible Commentary, by Cyrus Ingerson Scofield, , at sacred-texts.com
Book Introduction - Psalms
The simplest description of the five books of Psalms is that they were the inspired prayer- and-praise book of Israel. They are revelations of truth, not abstractly, but in the terms of human experience. The truth revealed is wrought into the emotions, desires, and sufferings of the people of God by the circumstances through which they pass. But those circumstances are such as to constitute an anticipation of analogous conditions through which Christ in His incarnation, and the Jewish remnant in the tribulation (Psalms 10-21, refs), should pass; so then many Psalms are prophetic of the sufferings, the faith, and the victory of both. Psalms 22 and 50 are examples. The former--the holy of holies of the Bible-- reveals all that was in the mind of Christ when He uttered the desolate cry, "My God, My God, why hast Thou forsaken Me?" The latter is an anticipation of what will be in the heart of Israel when she shall turn to Jehovah again (Deu 30:1-2). Other Psalms are directly prophetic of "the sufferings of Christ, and the glories which should follow" (Luk 24:25-27; Luk 24:44). Psalm 2 is a notable instance, presenting Jehovah's Anointed as rejected and crucified (Psa 2:1-3; Act 4:24-28) but afterward set as King in Zion.
The great themes of the Psalms are, Christ, Jehovah, the Law, Creation, the future of Israel, and the exercises of the renewed heart in suffering, in joy, in perplexity. The promises of the Psalms are primarily Jewish, and suited to a people under the law, but are spiritually true in Christian experience also, in the sense that they disclose the mind of God, and the exercises of His heart toward those who are perplexed, afflicted, or cast down.
The imprecatory Psalms are the cry of the oppressed in Israel for justice--a cry appropriate and right in the earthly people of God, and based upon a distinct promise in the Abrahamic Covenant
See Scofield - Gen 15:18, but a cry unsuited to the church, a heavenly people who have taken their place with a rejected and crucified Christ. (Luk 9:52-55).
The Psalms are in five books, each ending in a doxology:
1. Book I (Psalms 1-41).
2. Book II Psalms (42-72).
3. Book III Psalms (Psalms 73-89).
4. Book IV Psalms (Psalms 90-106).
5. Book V Psalms (Psalms 107-150).