It was requested that I write something for this group on the subject of the Book of Shadows in Wicca. I'm going to discuss the history of what we call the Book of Shadows first. And I'm sad to report that it isn't the mysterious shrine of knowledge steeped in the mists (or is that "mysts?") of antiquity that many believe it is. You see, the term, "Book of Shadows" was made up by Gerald Gardner himself. Actually, that isn't even correct. I was ripped off by Gardner himself. It is reported that when Gardner wrote the novel "High Magick's Aid" in 1949, he didn't include the concept of the book of shadows because he hadn't thought of it yet. It is reported by Gardner's High Priestess, Doreen Valiente that Gardner saw an ad for his novel in a local publication and across from it was an ad for a divination treatise called "The Book of Shadows." Gardner liked it and decided to name his grimoire after it. Which is fine. Except that Gardner compiled his grimoire from various sources including the Keys of Solomon, Aliestair Crowley's works with the OTO, Freemasonry, and Charles Leland's "Aradia, or the Gospel of the Witches," and then packaged it as an ancient text that was purportedly handed down to him from an "unbroken line" of witches stretching back into antiquity. And Gardner's High Priestess Valiente called him on his crap. When questioned, Gardner admitted "adding" to the original grimoire that was passed down to him.
There are early copies of some of Gardner's Books of Shadows and they are now in private collections. I myself have not read them. I do, however, have it on good authority that many of the practices that are now quite common in many Wiccan Traditions were not present in the practices of Gardner and his cohorts in the early days of what would come to be called Wicca. Some of you may know that Gardner's religion was called Witchcraft and a male adherent was called a "Wica." The word "Wicca" is attributed to the Llewellyn Publishing Company's wildly popular series written by Scott Cunningham in the early 1980's which described "Wicca" and introduced the concept that actual training and initiation were not necessary (cough).
Now, magickal grimoires have been around for centuries (http://www.spelwerx.com/grimoires.html). And honestly, as a Wiccan, it really doesn't bother me that my religion was invented by Gerald Gardner who assembled it from various ancient and not so ancient sources. Wicca has since gone through a whole series of changes that have led to the formation of many different, cohesive religious traditions that serve the spiritual needs of many people today. Gardner was a character to be sure. But just because he threw a bunch of things together that had never been brought together before and tried to package it as an ancient tradition, it doesn't mean that practicing the religion that was born from Gardner's efforts isn't valid. If it weren't valid for those of us who practice it, we wouldn't do it. And there are plenty of highly educated and very intelligent Wiccans out there, so I say Wicca is valid regardless of its originator's tendency to...erm, stretch the truth.
So because of how the term "Book of Shadows" came into being, I tend to call my personal book my grimoire. But if you want to call yours a "BoS," then by all means you would be in good company with many Wiccans out there.
In general, for solitary practitioners, the term "Book of Shadows" has come to mean a personal magickal journal. This is NOT what it means in Traditional Wicca. In Traditional Wicca, a BoS is a book in which pertinent information about that particular Tradition is kept. The book contains conventions, teaching materials, oathbound rites (the three degrees, etc.), liturgy, and things of the Tradition that should be passed on by High Priests and High Priestesses who were trained in the home coven and then go on to hive their own covens. In no way would it be appropriate to record "journal like" entries into a Traditional Book of Shadows. That's what coven journals are for.
In our coven journal, we record recipes (Trad cakes, coven oils, incense blends, May Wine recipes, etc.), elevation dates, magickal spell workings undertaken by the coven, ritual notes, circle casting poems, the words of the Gods that we receive through invocation, divinations, weddings, deaths, offices held by certain coven members, etc.
Now, a coven's Book of Shadows is different from a Tradition's Book of Shadows. The two are "halves of a whole," if you will. In my Trad, the "big" BoS is called the Blue Book. A coven's BoS would contain specific liturgies, conventions that the coven has added that are not part of the original Tradition (such as how to access the coven's astral temple, official oil or incense blends for elevations) or anything "new" that the coven adds as a permanent part of their practice. The reason that we keep Trad BoS's and coven BoS's is so future High Priests and High Priestesses can know what is of the Trad and what is of the individual coven, so that proper teaching can be offered to the students of the future and proper credit can be given to those who invent conventions that end up getting passed on.
Why am I telling you all this? Because I think it's a travesty that many solitary practitioners are not given historical information and therefore just incorporate phrases and other conventions into their Wiccan practices without benefit of knowing where they came from. And then when they meet trained practitioners, they can appear uneducated despite their efforts to educate themselves. And that's not fun. So there you go. That's where "Book of Shadows" came from and how the term is used in Traditional Coven Craft. If you want to call your personal grimoire a "Book of Shadows," now you can do so with full awareness!