Liber Brevior
The Liber Brevior is an "unofficial" Roman Catholic liturgical book used during the sung High Mass in force up to 1962 and revived in 2007 by Pope Benedict XVI. The "Liber Brevior" is essentially exactly the same as the “Liber Usualis" (used by Roman Catholic priests, Roman Catholic monks, nuns and seminarians for the Catholic Mass and Divine Office / Breviary up until 1962) except that it has been shortened from around two thousand pages to eight hundred by eliminating those parts not associated with the Mass (namely, the music from the "Breviarium Romanum"). It could be said that  the "Liber Brevior" is “the layman’s Liber” designed and intended to be used by small Catholic choirs. Like the "Liber Usualis", the "Liber Brevior" book is an "unofficial" book of the Catholic Church which combines in one volume, for the sake of convenience, the following two "official" books of the Catholic liturgy for the Mass: 1. The "Kyriale" which includes the usual eighteen settings of the "Ordinary (liturgy)" of the Mass (Kyrie, Gloria in excelsis Deo, Credo, Sanctus and Agnus Dei) as well as certain other music for the Mass (Asperges Me, Sursum Corda, final blessing response, etc.), and  2. the "Graduale Romanum" which includes the "official" music of the Mass Propers (Introit, Graduale or Tract (liturgy), Alleluia, Offertory and Communion). The "Liber Usualis" includes the two previously mentioned "official" liturgical books plus: 3. parts of the "Breviarium Romanum" (the spoken or recited readings of the Breviary or Liturgy of the Hours (also called the Divine Office), and, 4. parts of the "Antiphonary," "Responsory," "Psalterium," which are some of the music books associated with the "Breviarium." The Henri and Jules Desclée publishing company in Belgium and the Gregorian Institute of America in 1954 came out with the "Liber Brevior  to meet the needs of those who wished to sing Gregorian Chant at a sung Tridentine High Mass, but did not need those parts pertaining to the Divine Office / Breviary. In the back section of the book there is another set of Graduals, Alleluias and Tracts in a simpler Gregorian Chant tone arrangement of the same music. This book can be used with the traditional" sung Tridentine Latin High Mass of 1962 as reauthorized by Pope Benedict XVI motu proprio letter "Summorum Pontificum" published in 2007. References.