"Be filled with the Spirit, speaking to one another in psalms and hymns and spiritual songs." 
(St Paul, Eph. 5: 18)

General:

A History of Gregorian Chant by Alison Hope.
Gregorian Chant: A Barometer of Religious Fervor by Stephen Thuis, O.S.B.
Gregorian Chant Notation.
Papal Legislation on Sacred Music, Principle Points this Century.

Full texts of papal documents concerned with sacred music:

Encyclical Musicae Sacrae (Pope Pius XII).
Motu proprio Inter Sollicitudines (Pope Pius X).
Instruction De Musica Sacra (Sacred Congregation of Rites).

(The following overview has been reproduced from the Abbey of Solesmes website: www.solesmes.com/anglais/gregorian/greg__home.html)

The story of gregorian chant

Gregorian Chant is a musical repertory made up of chants used in the liturgical services of the Roman Catholic Church. In fact, the liturgical tradition which the Church has bestowed on us is a vocal, monophonic music composed along with Latin words coming from sacred texts. This is why Gregorian Chant has often been called a "sung Bible". Linked intimately to the liturgy in this way, the goal of the Gregorian melodies is to favor spiritual growth in everyone, reveal the gifts of God, and the full coherence of the Christian message.

What we call Gregorian chant today first appears distinctly in the Roman repertory of the fifth and sixth centuries. Its care and perhaps some of its composition was in the hands of a group of ministers in a specially dedicated service to the Roman basilicas, the schola cantorum. Gregorian chant also appears to have been an aural music, that is, transmitted by ear and committed to memory - like other music of the world at the time.

In the second half of the eighth century, the political rapprochement between the French kingdom of Pepin and Charlemagne, and the papacy, widened the Roman liturgy's field of appreciation. The French crown decreed its adoption throughout the kingdom. It is at this time that the written records which have come down to us begin to appear first in France, then all over the Empire and beyond. Despite wide graphic differences, their uniformity of content clearly records a single reading of an unbroken tradition.

The texts (words and some musical notations), committed to writing in books, become an official reference text. The general allure of the Roman chant with its modal architecture was of great attraction to Gallican musicians. They dressed it, however, in a completely different way. The term "Gregorian chant" was first used to describe this hybrid of Roman and Gallican chant.

At first, written records served as memory prompts with just the artistic directions for correct interpretation and performance. The musical tones were still taught by ear and transmitted from memory.

But with the gradual increase of pitch indications in the manuscripts came a corresponding decrease in the interpretive directions, and with it accordingly, a decrease in the role of memory. As a result, Gregorian chant fell into complete decadence by the end of the Middle Ages: the manuscripts offer little more than a "heavy and tiresome succession of square notes". The Renaissance brought with it Gregorian chant's coup de grâce. The melodies, which show the correct reading of the literary text by highlighting keywords and phrases, were "corrected" by official musicologists - the long vocalises, for example, reduced to a few notes each. Worse, the words, literary compositions which are the official text of the Roman liturgy, and that constitute a lyrical catechism, were also officially "corrected" against a verbatim reading of the Vulgate Bible. The mangled result which persisted for two hundred years is generally known in English as "plainsong".

In 1833, a young priest of the diocese of Le Mans, Dom Prosper Guéranger, undertook the restoration of benedictine monastic life on the site of an old priory at Solesmes, after forty years of interruption due to the French Revolution: he seized upon the restoration of Gregorian chant with enthusiasm. He began by working on its execution, asking his monks to respect the primacy of the text in their singing: pronunciation, accentuation and phrasing, with an eye to guaranteeing its intelligibility, in the service of prayer. Dom Guéranger also placed the task of restoring the authentic melodies into the hands of one of his monks.

The handwriting, in "thin flyspecks", of the original manuscripts was indecipherable at the time. But the invention of photography soon brought unforeseen benefits with it. Little by little, an incomparable collection grew at Solesmes, facsimiles of the principal manuscripts of the chant contained in the libraries of all Europe: the Paleography of Solesmes.

The musical forms of Gregorian chant

THE PROPER CHANTS OF THE MASS


The Propers are the pieces whose text varies according to circumstances. The principal pieces of the Propers are the Introit, the Gradual, the Alleluia, the Offertory chant, and the Communion chant.

The Introit:
The Introit accompanies the entrance procession of the celebrant and his ministers, and brings the faithful into the particular mystery being celebrated: it "sets the tone" of the day, feast or occasion.

The Gradual :
The Gradual is one of the reading responses. It is constructed from a form of psalmody with refrain. Originally, the congregation responded with a simple formula to a soloist who sang the verses of the psalm one by one. But in the fifth to sixth centuries, a musical enrichment led to a curtailing of the literary texts.

The Alleluia:
"Praise the Lord" is the literal translation of this Hebrew word. At Mass, it was originally a chant reserved for Easter Day alone. From there its use was extended to Eastertide, then to Sundays of the year, weekly celebrations of the Resurrection.

The Offertory:
This is not just a "functional" chant but more of an accompaniment to the ceremonies, a sumptuous "musical offering" of sorts.

The Communion:
The purpose of this chant is to accompany the procession of those distributing communion. Also, it frequently seeks to create a synthesis between the liturgy of the Word and the liturgy of the Eucharist.

THE ORDINARY OF THE MASS

Other than the Proper chants, whose text varies according to circumstances, the celebration of the Mass includes chants whose texts are fixed, independent of the day or feast.

The Kyrie:
Kyrie eleison is a Greek formula by which the faithful "acclaim their Lord and implore his mercy." Today this chant is placed at the beginning of the Mass, as part of the penitential rite, preparing the faithful for the celebration.

The Gloria:
This hymn of Eastern origin may date from as early as the second century.
In the Roman liturgy the Gloria originally came into use for the midnight Mass of Christmas only. Later it was steadily extended to the great feasts of the year and to Sundays.

The Sanctus:
At the beginning of the Eucharistic Prayer the Sanctus is introduced by the great recitation of the Preface. The Sanctus is the "hymn of the Seraphim", heard in the Temple of Jerusalem by the prophet Isaiah. It invites the Church on earth to join in the liturgy of heaven.

The Agnus Dei:
This is the chant which accompanies the breaking of the Bread which has just been consecrated, a necessary breaking which preceeds its distribution at the communion of the faithful. The congregation puts the time between the fraction and the communion to good use "by greeting with homage and humble supplication the one who has been made present under the appearance of bread".

THE DIVINE OFFICE

This great daily prayer of the Church consecrates the whole of human time by means of divine praise. Seven times a day, and again once every night, the Christian community gathers together to celebrate this liturgy which is basically composed of the psalms.

The Antiphons:
The chanting of a psalm is framed by a brief piece called an antiphon. Sung for its own sake, it introduces and concludes the psalm. Like the propers at mass, proper antiphons of the offices change with the feast or day and give each of the psalms they accompany a particular reading - as the psalm is chanted to the same musical tone the presence of the antiphon's text subtly descends into the whole psalm on sundays and feasts.

The Responses:
These are chants which come between the readings from the Bible and from the writings of the Church Fathers during the Night Office. On great feasts, they follow the reading at first vespers. They are, par excellence, meditative chants, contemplative musical commentaries of the sacred text.

The Hymns:
The most popular pieces of the office are undoubtedly the hymns. Their importance in the Western liturgy was recalled by Vatican II. The hymn sets the tone and helps the faithful enter into the liturgical season or the particular mystery being celebrated. Often it is a simple and melodious composition.

CONCLUSION

At first hearing, Gregorian chant might seem monotonous. Undoubtedly it disconcerts our modern ears, accustomed to more contrasted music, but often less profound. In reality the Gregorian repertory is a complex world which unites several centuries of musical history. It is in fact a world of astonishing variety which mysteriously approaches nearly delirious enthusiasm as well as the most delicate interior things. It is a paradoxical world where music blooms in silence.