The Book of Shadows is a Witch's journal. It is something she holds very dear to her heart- it is a fragment of her memory, an extension of herself. Witches record a list of spells that they worked on, or intend to work on. Correspondences that one uses while working on a spell. Rituals and meditation performed and how they went. It can also hold a list of supplies nought and in stock. Young witches use a BoS as a notebook to record all that they have learnt. Some people also record their dreams in their BoS. There is no hard and fast rule as to what you need to pen down in there.