The Liber Usualis is one of the most important publications of liturgical chant in the Roman Rite. Containing music for both Vespers and Mass for all Sundays and Feasts, it appeared in numerous editions between 1886 and 1961 and aimed to restore the original melodies of Gregorian chant, which had been subject to considerable simplification in the 1570 Missale. Since the Vatican II revision of the lectionary its place has been taken by the 1990 Roman Missal.
It is now available at Liber Usualis online.
View the Wikipedia article on Liber Usualis.