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.