What is a grymoire?

Answer 1

A collection of witchcraft lore, handed down through a family, clan, or coven. It is not very different from the family book of recipes that Grandma used to have.

The Grymoire contains basic knowledge all witches share, descriptions of, well, creatures, and contains spells, collected through the generations.

The book would be, literally, the source of the witches power. Today we attach a more metaphysical meaning to that. I don't know how accurate that would be. But, the book would've been protected.