The principal source for music for the Mass. It contains all the Propers, the Kyriale (chants for the Ordinary), chants for the people's responses, the priest's prayers, and the readings, and music for the Litany of the Saints and the Te Deum.
An edition of the Graduale Romanum, supplemented with the notation from the 10th-11th Century Swiss manuscripts of St. Gall and Einsiedeln (printed in red below the staff), and the 10th-Century French manuscripts of Laon (printed in black above the staff).
This book is not quite a full altar missal with chant notation, but it contains the music for most of the priest's parts, the Prefaces written out in both solemn and simple tones, the Eucharistic Prayers, music for the seasonal solemn blessings at the end of Mass, the Good Friday prayers, and the Exsultet.
A bilingual hand missal for Sundays and major solemnities and feasts, augmented with chant. It is available in variants for several languages; the English one contains the ICEL translations of most of the Mass, but literal translations of the chant texts themselves. It does not contain the texts of the readings from Scripture.
The hymnal for the Liturgy of the Hours, which, in addition to the hymns, contains some antiphons, some responsories, and the ornate tones for Ps 94(95).
ISBN 1-55725-040-0, 2-85274-095-8, and 2-85274-132-6
The Liber Cantualis contains music for the people's responses at Mass, selections from the Kyriale, the Requiem Propers, music for Compline, and a selection of popular hymns, antiphons, and other chants. It is perhaps the single most versatile book to use when beginning to teach chant, or for use in the pews.
The Kyriale is the section of the same name from the Graduale, bound separately.
The Cantus Selecti contains a great variety of devotional hymns and antiphons, mostly intended for use at Adoration of the Blessed Sacrament but arranged according to the liturgical year. It makes a good supplement to the Liber Cantualis for building a fledgling schola's repertory.
The old Liber Usualis, which had gone out of print after the promulgation of the new Missal, has been picked up and reprinted by a small publisher in Montana, which publishes a lot of "traditional" (read pre-conciliar) books. It is expensive but well worth having, since it has music for the Liturgy of the Hours as well as for the Mass, the chants for Advent, Christmas, Lent, Easter, and most saints' days haven't changed, and it contains a guide to the ecclesiastical pronunciation of Latin.
Dom Eugéne Cardine, one of the monks of the Abbey of St. Peter, Solesmes, France, has written extensively on interpreting the notation of ancient manuscripts, and performance practice using them as a guide; two of his books are easily available in English translations.
(no ISBN listed)
This is Dom Cardine's introductory textbook. The translation is not the best; it has many infelicities of phrase, what seem to me to be questionable usages, and annoying inconsistencies in spelling. The book itself, I think, would read better as a companion to lectures or rehearsals with an experienced teacher; it is sometimes difficult to figure out how to apply the lessons if all one has is the book.
Dom Cardine's second-level textbook; it follows on from Beginning Studies, and is a near-mandatory companion to the Graduale Triplex, since it explains the meaning of the ancient notation. The translation is much better than that of Beginning Studies, and it reads more easily; however, there are still places where the book would make more sense were one simultaneously attending rehearsals with Dom Cardine.
Other writers have also contributed useful secondary sources:
The translation of Gregorian Semiology was done by Robert M. Fowells, who teaches at California State University. He has also published this book, which is a simplified version of part of the material in Gregorian Semiology. In some ways it is also a commentary on that book's contents, explaining some of Dom Cardine's terms. It also contains a sample of chant copied from the Graduale Triplex, with comments on how to read the ancient notation.
This delightful monograph was published in 1958, before the bulk of Dom Cardine's work. That notwithstanding, it is a comprehensive and well-researched reference work that gives an excellent overview of the chant and research into performance practice. In some ways it even seems to anticipate Dom Cardine.
This book is a more recent (1993) reference; it does not displace Apel but adds to his work. It is not conducive to being read straight through in the way Apel's book is, but its scope is wider.