A Grimoire is a magical manual or a magician’s instruction book. It is much more formal than a Book of Shadows.
While a Book of Shadows is more journal-like, a grimoire is specifically a book of instructions for performing magickal tasks, creating magical objects and symbols and interacting with spirit beings, such as angels and demons.
It is generally believed that the word grimoire came from the Old French word grammaire, a term used to describe Latin texts. Later the word was used in colloquial French to describe something that was difficult to understand.
Check out sacred-text.com’s index of grimoires at http://www.sacred-texts.com/grim/index.htm