What is a Grimoire? In its simplest form, it’s a notebook full of magic spells.
It’s where a wizard keeps his most precious knowledge: spells he’s learned and gathered over the years; snippets, incantations, assorted poems and sketches. It’s where he goes when he’s trying to solve a problem, brush up on an old technique, or document his attempts to learn something new.
As the great Sussman said:
If you have the name of a spirit, you have power over it.