The Fragile Path: Testaments of the First Cabal is a sourcebook for Mage: The Ascension. Like The Book of Nod for Vampire: The Masquerade, The Fragile Path is presented as a book that exists in-universe and could be used as an in-character prop in a game.
The Author page (p. 2) lists all the in-character authors. The Acknowledgments (sic) on the same page is largely an in-character presentation of the real-world people responsible for creating the book, with several in-character references thrown in.
Mage: The Ascension books
Stephen C. Brown