The Book of Shadows is basically a Witches Diary.  It will contain all of her imformation on things important to wicca, such as information on the Goddess and God, Sabbats, Esbats, Chakras, Meditation, Herbs, Incense, Astrology and many more things.  It is also used to record the magickal workings of a witch.  The Book of Shadows is a sacred item of any Pagan - and therefore should be treated with the upmost care and respect.