This is Cantus Selecti from the Solesmes Monks, published in 1957. It contains difficult to find scores for many incidental Gregorian chant melodies. 
     “Since the refounding of the monastery of St. Peter of Solesmes in 1833 under Dom Prosper Gueranger, this Benedictine monastery on the River Sarthe in western France has set the world standard both for the performance of Gregorian chant and the authenticity of the music itself. Commissioned by Pope Pius X to research the role of chant in liturgy and to edit books on chant that reflect this research, Solesmes enjoys an enviable reputation. Its work in liturgical reform and in the current revival of Gregorian chant has been likened to that of the great abbey of Cluny in the Middle Ages… The groundbreaking work by the monks of Solesmes has contributed to today’s resurgence of interest in Gregorian chant as the foundation of Western music.” (Paraclete Press)