Gregorian Chant:

In what liturgical books can one find the Gregorian Chant used in the EF Mass?

The Liber Usualis  (1962) contains all the chants for each day of the liturgical year.  It also contains all the readings, prayers, and rubrics of each Mass.  It can be purchased from Biretta Books.

Simpler versions of the official chants may be used by smaller choirs, as long as the text remains unchanged.  Once source for simpler, psalm-tone versions of the graduals, alleluias and tracts (usually the most elaborate chants for Mass) is the Liber Brevior.  Another simple version of the propers is known as the “Rossini Propers,” set to modern notation.

If the Chant propers found in the Liber Ususalis seem intimidating for smaller choirs, can a choir sing a simplified psalm tones setting of the propers for the liturgical year?