The psalms go back a long, long way. They are attributed to King David in the same way that Gregorian chant is attributed to Pope Gregory: a combination of history and mythology. By the time the young Jesus was learning to sing the psalms, he was being formed in a tradition that was already a thousand years old, and the psalms became a basic vessel for his self-understanding. Again and again Jesus responded to questions about himself and his mission through reference to the psalms, most poignantly in his last words on the cross: in Matthew, "My God, my God, why have you abandoned me?" (Psalm 22:1); and in Luke, "Father, into your hands I commend my spirit" (Psalm 31:5).
The tradition of monastic psalmody was very clearly in place by the third century A.D., when the curtain rose on the Desert Fathers and Mothers whose ascetic experiments in the deserts of Syria and Egypt constitute the most powerful and sustained exploration of the path of inner transformation ever to have arisen in Christianity. Those haunting psalm tunes became the lifeblood of Desert spirituality and laid the groundwork for nearly two millennia of Christian spiritual practice. "The psalms are the path you must follow," said St. Romuald, the 11th-century founder of the Camaldolese Benedictine order, "--never leave it."
So as Father Theophane pointed out, when we work with the psalms, we're working with a tradition of sacred poetry that has been hallowed by usage. The psalms are made holy by the fact that so many people on the Christian path have used them, loved them, processed their own spiritual journeys through them. When we sing the psalms, we are walking on a well-trod path.
The easiest place to find the psalms is early in the Bible--which means they are a tradition common to both Jews and Christians. They are the sacred poetry of ancient Israel, 150 of them altogether. Some of these are brief (two or three verses); others go on for pages.
Despite their many moods, the one thing that all psalms have in common is that they are songs. That's what the word "psalm" means. Many of these songs were originally intended for use at festivals and in synagogue worship. Others are more introspective, clearly for private reflection and devotion. In Christian monastic tradition, singing the psalms at regular intervals throughout the day--a practice known as the "Divine Office"(Opus Dei)--furnishes the basic rhythm of the monastic day and the grounding for both daily work and contemplative prayer.
While you can find the psalms in the Bible, of course, there are other places as well. A book of psalms is called a psalter, and there are many modern psalters available in a wide variety of translations. In my own denomination, the Episcopal Church, an excellent resource is the 1979 Book of Common Prayer, which offers accurate and sensitive translations of all of the psalms. It's available for perusal in most any Episcopal church pew, or can be ordered directly from the Episcopal Church Publishing Company.
Among Roman Catholics, the Grail Psalms are highly in favor. This paperback psalter, was recently updated to reflect more inclusive language, and is easily and inexpensively available from GIA publications in Chicago. It also serves as the "libretto" for most of the musical settings currently being developed in Catholic monasteries.
For many centuries Latin was the universal language of Catholic Christianity and Gregorian chant was the universal musical language, This all changed rather dramatically in the early 1960s when the famous Council of the Roman Catholic Church, known as Vatican II issued a firm directive toward returning the language of worship to the vernacular. In the wake of that directive, the old Gregorian tradition was basically shelved as composers began to develop new musical settings for the psalms, hymns, and Mass texts in contemporary language versions.
In some ways this was a great loss as the Gregorian tradition is extraordinarily beautiful. But in other ways it's been a gain, resulting in a new infusion of creative energy into the singing of the psalms, as well as making the practice far more accessible. It allows us to re-encounter the psalms as they really are--not just aesthetic artifacts of some other religious era but timeless outcries of our own questing hearts and spirits.