There are some other interesting finds at this site if you search around; for instance, Dom Joseph Pothier's Les mélodies grégoriennes d'après la tradition
* A short work on the chants of the Kyriale (in Italian, «I canti del Kyriale»), the major interest of which is that it includes most of the chants of the Kyriale Simplex (browse the pdf to find them in the correspoding sections).
* A few pages (unfortunately, very few) of the 1935 Antiphonale Missarum of the Ambrosian Rite.
* Only marginally related to chant: from Archive.org again, two other books of Apel's: The «Harvard Dictionary Of Music», and «The Notation Of Polyphonic Music, 900-1600».
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