Grimoires

Simply put, any time a magical compendium is written down to be passed on to later generations, the result is a grimoire.

It is said that today, the most famous grimoire in the world has a history of over 2000 years and is still in circulation, acquiring new alliance members even now.

It is forbidden to speak the true name of that grimoire, so it is simply called "Book".

Knowing this doesn't help us whatsoever, but he's definitely referring to the bible here, it's the most printed book in the world, thousands of years old, and the name is derived from the greek word for books.

Magical compendiums are narratives

Magical Compendiums, or systems of magic, are stories or narratives that validate a set of beliefs. They gain more power the more people buy into them but they can also belong to just one person.

The example given by the author for a grimoire, aka a written magic compendium is very clearly the bible, so massive religions and ideologies fit the definition, but so does Maria's coping mechanism to deal with family issues, to name one.