The prayer book in this article is most likely a book with liturgical prayers. The text that the article discusses was just written on a leaf in that book (like

The prayer book in this article is most likely a book with liturgical prayers. The text that the article discusses was just written on a leaf in that book (likely one of the opening or ending ones). It's quite common to find in books of the era notes about personal or communal events written on what were blank (or mostly blank) pages.