Latin Lyrics for Gregorian Chants Does the Catholic Church have a particular book that contains the Latin lyrics for Gregorian chants? I suppose most of these chants are sung as part of a liturgy.

One such chant is named Communio: Omnes qui in Christ (video link)

I suppose it's sung for Communion.
The books you want are the "Kyriale" and the "Graduale".  The former contains the words and music for the ordinary of the mass (Kyrie, Gloria, Credo, Sanctus, and Agnus Dei), and the latter contains the words and music for the variable parts of the mass (Introit, Gradual, Alleluia verse, Offertory, and Communion verse).