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 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.

It is because that is such a true description of them that the Psalms have always proved to be a great source of solace and encouragement to God's people throughout the centuries - both the children of Israel and the members of the Christian Church.

Here we are able to watch noble souls struggling with their problems and with themselves.  They talk to themselves and to their souls, baring their hearts, analyzing their problems, chiding and encouraging themselves.  Sometimes they are elated, at other times depressed, but they are always honest with themselves.  That is why they are of such real value to us if we also are honest with ourselves.  (D. Martyn Lloyd-Jones, Spiritual Depression, 9).