A grimoire is a textbook. It's not your ordinary sort of French or geography textbook, though, full of garçons and global warming.
No, a grimoire is a textbook of magic and sorcery.
What? Oh, well, sorcery is when you use the power of other-worldly creatures - ghosts, vampires etc - to get your own way.
This is almost always a VERY BAD IDEA INDEED and I wouldn't recommend it. In fact, any plan you might have which involves trusting a vampire is very likely to end in tears.
Magic, on the other hand, covers loads of other stuff, too. The power of magic can be derived from perfectly harmless sources such as porridge pots, mirrors, or, of course, words:
Hang on! Where did that purple frog come from?
Word To Use Today: grimoire. This word is wonderful to say, especially if you have a cloak to swish or a moustache to twirl. Try saying I must consult my grimoire in a mysterious Transylvanian accent - it's much more impressive than saying I'll look it up on Google.
Grimoire is from the French word grammaire, which means grammar, and comes from a time when all writing seemed magical. Grammaire is from the Greek word gramma, which means letter (as in alphabet, not as in bank-statement).
Ribbit! Ribbit! Ribbit! Ribbit! Ribbit! Ribbit! Ribbit! Ribbit!