A Book of Shadows is a concept of the Neopagan religion of Wicca, often abbreviated as BoS in writing. A Book of Shadows contains guides on rituals, information on herbs and crystals and notes on astrology, divination and the like.
The Book of Shadows concept goes back to Gerald Gardner and his teachings within the Bricket Wood Coven. According to Doreen Valiente, a witch who soon rose to become a High Priestess of Bricket Wood Coven, Gardner had borrowed extensively from the famed occultist Aleister Crowley, from the book Aradia: Gospel of the Witches and the Key of Solomon. Valiente set out to re-write a lot of the Book of Shadows to retain a more Wicca-based tone of voice rather than Crowley-esque.
Today, a lot of solitary practitioners use it as a guiding journal and write down their magickal undertakings in everything from lucid dreaming to cleansing stones. This is normally not passed down from student to teacher as a Book of Shadows might be in a coven.
What about the Grimoire?
The Grimoire is the Witch’s magickal book. Where as a Book of Shadows contain ALL information related spells and rituals, such as correspondence lists and Grimoire is usually only filled with the magickal working of the ritual alone. How the books are treated also differs. A Book of Shadows is often (these days) not written in encrypted style but in plain native tongue – a Grimoire is more often than not written in deep symbolism and maybe even a crafted language or written by using runes; all to prevent unworthy eyes from feasting upon the contents. Another difference is that a Book of Shadows can be handed down to other members of a coven or to other Wiccans, whereas a Grimoire is not expected to be handed down in the same manner.