A Book of Shadows is a magical tome kept by good witches, containing spells, potions, information, past experiences and general notes about magic. The most important and powerful book in existence is the Book of Shadows owned by the the Warren line. Begun by Melinda Warren during the late seventeenth century, the tome contains centuries worth of knowledge and is blessed with powerful magic. As a rule, written by the the Elders, whitelighters are to have their charges keep their books in a sacred altar room, though Leo Wyatt did not apply the rule to the Charmed Ones, who mocked the concept.