There's a discount available at Lulu.com for 15% off print books, which may come in handy for ordering some of CMAA's editions. The coupon code PUBLISH15 is available until the end of July 17, 2020.

performing and reference editions:
Chants of the Church
Communio with English Verses
Evangelia Cantata (notated Gospels for Ordinary Form Sundays)
Graduale Romanum 1961: volume one and volume two
Kyriale Romanum
Officium Majoris Hebdomadae et Octavae Paschae cum cantu (1923 Holy Week book)
Versus Psalmorum et Canticorum (verses for EF entrance and communion antiphons)

instructional books:
Gregorian Chant for Church and School
Ward's Music First Year, Music Second Year, Music Third Year, Music Fourth Year, and Advanced Studies in Gregorian Chant
A New School of Gregorian Chant (Dom Johner)

more books about the liturgy and sacred music:
A Dictionary of the Psalter (Britt)
Questions and Answers about Sacred Music, in English and Spanish
The Hymns of the Breviary and Missal
Breviary Hymns of Old Uses
The Rhythm of Plainsong (Gajard)
The Spirit of Gregorian Chant (Pierik)

Happy reading and singing!