See this online ebook:

  • The Fundamentals of Gregorian Chant: A Simple Exposition of the Solesmes Principles Founded Mainly on “Le Nombre Musical Grégorien” of Dom André Mocquereau, by Lura F. Heckenlively (Tournai, Belgium: Society of St. John Evangelist, Desclée & Co., 1900). Available at Open Libraryand Internet Archive.

On the Feast of Pope St. Gregory the Great in the Eastern Catholic churches and in the Latin church (Extraordinary Form of the Roman Rite). For other legally free ebooks, you may access the List of Free eBooks (Arranged by Title) and the List of Free eBooks (Grouped by Subject).