The Book of Law was a book of magic and spells written by the former Jedi Knight Allya.
Originally the The Book of Law was meant to be moral a guide to teach Allya's female descendants in safe and sane uses of their magic. Since Allya was once a Jedi Knight, she based some of the text on the Jedi Code.
Althrough the message in each book was based on Allya's original message, each clan added its own discoveries and spells to their copy. Some clans even took Allya's original message and replaced it with various interpretations, such as Book of Shadows done by the Nightsisters.