A few years ago, I was fortunate enough to stumble upon a copy of the
Liber Usualis at a used bookstore. Reprints that I've seen cost more than $100, but Musica Sacra has been kind enough to put a pdf of the whole thing online
here. The
Liber Usualis contains all the chants for the major liturgies of the Roman calendar, along with a brief introduction to how to read and interpret the notation Gregorian Chant is written in: a four-line staff with square notes, which is the predecessor of our modern notation.

Thus far, my aspirations to learn more of the chants for the office and mass haven't amounted to much--aside from chanting the gradual for the nuptial mass, "Uxor Tua", in the prelude to my wedding.
However, as part of our Advent preparation, my wife and I have been praying vespers together, so for the final week, we are learning and chanting the great "O Antiphons" together. These antiphons, each invoking Christ under a different title, are used with the Magnificat at vespers for December 17-23. According to
Wikipedia, Boethius makes a slight reference to them, indicating that they date from at least the 5th century. They were in use in Roman liturgy by the eighth century, and, one may presume, spread from there.
December 17O Sapientia, quae ex ore Altissimi prodiisti,
attingens a fine usque ad finem,
fortiter suaviterque disponens omnia:
veni ad docendum nos viam prudentiae.
December 18O Adonai, et Dux domus Israel,
qui Moysi in igne flammae rubi apparuisti,
et ei in Sina legem dedisti:
veni ad redimendum nos in brachio extento.
December 19O Radix Jesse, qui stas in signum populorum,
super quem continebunt reges os suum,
quem Gentes deprecabuntur:
veni ad liberandum nos, jam noli tardare.
December 20O Clavis David, et sceptrum domus Israel;
qui aperis, et nemo claudit;
claudis, et nemo aperit:
veni, et educ vinctum de domo carceris,
sedentem in tenebris, et umbra mortis.
December 21O Oriens,
splendor lucis aeternae, et sol justitiae:
veni, et illumina sedentes in tenebris, et umbra mortis.
December 22O Rex Gentium, et desideratus earum,
lapisque angularis, qui facis utraque unum:
veni, et salva hominem,
quem de limo formasti.
December 23O Emmanuel, Rex et legifer noster,
exspectatio Gentium, et Salvator earum:
veni ad salvandum nos, Domine, Deus noster.
Read the first letters of the titles of Christ, and you'll discover that those clever monks arranged them to spell "Ero cras": "I will be (i.e. arrive) tomorrow".