Here are some few things that are kept in a Book of Shadow (BOS). Remember that not all BOS are the same. This is based on what you are studying. For Wicca, notes on the holidays, any rituals you did, how you felt after and during the ritual. You can included stones, herbs, potion recipes, spells and even tea recipes or whatever you want related to what you are studying. You can also include your dream diaries with your own interpretation. If you like, you can also include the different moon phases in your BOS.
Good luck to you.