A textbook of magic.
It is most commonly believed that the term grimoire originated from the Old French word grammaire, which had initially been used to refer to all books written in Latin. By the 18th century, the term had gained its now common usage in France and had begun to be used to refer purely to books of magic, which Owen Davies presumed was because "many of them continued to circulate in Latin manuscripts". However, the term grimoire also later developed into a figure of speech amongst the French indicating something that was hard or even impossible to understand. It was only in the 19th century, with the increasing interest in occultism amongst the British following the publication of Francis Barrett's The Magus (1801), that the term entered the English language in reference to books of magic.
“Never commit to memory what can be easily looked up in books”
― Albert Einstein.