Divinum Officium

The Roman Breviary Online

The Liturgia Latina Project

The Divine Office, Mass, Sacraments, and Other Rites In Latin and English

The Liturgical year — by Dom Guéranger

The Sacred Liturgy

A short Catechism of the Mass

Sancta Missa — Tutorial on the Tridentine Latin Mass; Sacred Music of the Liturgy

The Holy Sacrifice of the Mass

Thesaurus Precum Latinarum

The Church's Year

Laudate Dominum — Traditional Rites and prayers

Per Ipsum — Daily Latin Mass Readings

GregoBase​ — A database of gregorian scores

Gregorian books — Propers, Hymns, chant recordings

Gregorian Chant

Gregorian Musical Forms — Abbaye Solesmes

Musica Sacra — The Church Music Association of America

The Sacrifice of the Mass — by M. Gavin, S. J.