We've added a helpful guide to the Liber Usualis to our resource collection on the CMAA website.

"Being at Ease with the Liber Usualis", a typewritten book designed for seminarians and religious students, is an introduction to the role of chant in the liturgy and to the genres of chant found in the famous anthology. Written in 1951 by Fr. Ermin Vitry, OSB, of Saint Meinrad Abbey, who taught at Mount Saint Mary's Seminary in Emmitsburg.

https://media.musicasacra.com/books/vitry-being-at-ease-with-the-liber-usualis.pdf

Thanks to the librarians at St. Patrick's University and Seminary in Menlo Park, California, for scanning the book and sharing it with CMAA.