The book of shadows or grimoire is used to record your magical workings and strictly speaking may not be considered a tool by all. Traditionalists tend to use the term grimoire and Wiccans book of shadows or BOS for short.
Despite popular movies and television shows, there is no one single book of shadows. A book of shadows, or BOS, is a Wiccan's or Pagan's notebook of information. It usually contains spells, rituals, correspondence charts, information about the rules of magic, invocations, myths and legends of various pantheons, etc.
Sometimes information in a BOS is passed along from one Wiccan to another (and in a coven setting, there may be a coven BOS as well as individual members' books), but you can create your own with a little bit of effort. A BOS is a very personal thing, and should contain the information you find most important.
-The Crafty Witch