Kelvin Smith Library
The Liber Usualis is the principal compendium of Gregorian chant. It contains the Ordinary of the Mass and the Office, and the Proper of time and of the Saints, including prayers and readings. The Kulas Music Collection includes various editions (listed below).
Usually, when people say "Latin Mass", they mean the Tridentine / pre-Vatican II rite. However, the normative form of the Missal of Paul VI is Latin, and it is performed in that language on occasion. These resources will make that possible, or will facilitate adding Latin to the Ordinary Form Mass.