ORGANIST VOLUME I, 388 pages long, contains three (3) different harmonizations of the the Gregorian Kyriale: 1.) Achille P. Bragers, LOW KEY; 2.) Carlo Rossini, MIDDLE KEY; and 3.) Nova Organi Harmonia, HIGH KEY.
Nova Organi Harmonia accompaniments were created by Flor Peeters, Msgr. Jules Van Nuffel, Msgr. Jules Vyverman, Marinus de Jong, Gustaaf Nees, Henri Durieux, and Edgard de Laet. St. Edmund Campion Missal & Hymnal for the Traditional Latin Mass: http://www.ccwatershed.org/Campion/
Ordinarium Missae Accompaniments, Keyboard Accompaniments for the Mass Ordinary