Most practitioners know what a book of shadows is and how to craft and use one. But its little discussed counterpart is the mirror book. Where a BOS is used to record your spells, rituals and things directly related to your practice of Wicca and magick, the MB is for everything that falls outside of that: your musings, your explorations, your thoughts, concerns, ideas, hopes, fears, dreams. The MB is your journal where your BOS is your spell book, if you will.
But the MB isn’t like a normal journal. It should only be used in conjunction with the BOS to record things that you experience, feel, discover, discard within your practice. It is your place to reflect upon strengths and weaknesses and determine which course of action to take. It can also be used to record dreams, or notes about your particular passions within the craft. It is literally your mirror. The intervals at which you write in the MB are up to you, though commonly it is written in each day, at least once within each phase of the moon, or once every ritual or spell practiced.
Acknowledging what you feel and think is a key part of practicing Wicca, and part of that acknowledgment requires writing things out. The mirror book and the book of shadows are two sides of the same coin; one is not complete without the other.