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".

Magical compendiums are narratives

Magical Compendiums, or systems of magic, are stories or narratives that validate a set of beliefs. By having people who believe in them they shape their actions and in turn the world, so a compendium shared by many people is all that more impactful for it, but a belief shared by just one person can still be very powerful for that 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.