The Book of Shadows (abbrev. BoS), sometimes Latinized to 'Liber Umbrarum', comprises one half of the accumulated lore and liturgy which is the core of British Traditional Witchcraft (or BTW). Originally named by Gerald Gardner, the founder of the BTW revival movement, the Book of Shadows went through several iterations before the collaboration between Doreen Valiente and Gardner resulted in the core version commonly found among BTWs today. The source materials used to fill in the 'gaps' in the lore that was received from the hereditary tradition which initiated Gardner were taken from a diverse number of sources, with influences as far ranging as THE KEY OF SOLOMON to the writings of Aleister Crowley. The textual study of the manuscripts which gave rise to the Book of Shadows is a favorite area of investigation for Craft Historians today. An excellent writeup on the subject, done by Stewart and Janet Farrar, can be found in the second half of THE WITCHES' BIBLE (Phoenix Publishing, Inc.; ISBN: 0919345921; August 1996).
According to custom, after Initiation, a new BTW witch will copy out the Book of Shadows by hand. This serves two purposes -- first, that the new witch has a copy of the lore appropriate to their degree for their very own, and second, that in the process of manually transcribing the BoS the lore will become imprinted on the subconscious level and thereby aid in the assimilation process which follows every initiation. Romantically, there is liturgical reference in some Traditions to 'keeping a book in your own hand of write' for the express purpose of protecting the identity of your covenmates from religious persecution from the Christians, but this is obvious romanticism and not based in either modern-day practicality nor factual historical evidence.
In most of the Traditions of BTW, the Book of Shadows is considered to be inviolate, in the sense that anything which is added to the BoS is supposed to be passed along, and nothing can be removed from the BoS that a new witch was passed. Ever. The strength of this custom varies by Tradition and family line, but in general it is regarded as sound. This has led to the current practice for witches to keep not one, but two books, one which is the actual tradition's Book of Shadows, and the second which functions as a personal grimoire or magical journal. The advantage to doing so is immediately apparent, since even three or four additions per generation of witches to the Book of Shadows would soon result in a BoS that was measured by the ream, not by individual pages. Half in jest, comments are made in 2002 about 'Ye Olde Suitcase of Shadows'.
The title of the Book of Shadows has been appropriated by the eclectic community in an attempt to capture some of the mystique and aura of history that the title tends to evoke. The truth of the matter is that the Book of Shadows is a very recent invention, since any peasant tradition of witchcraft prior to the 1800's would tend historically to be rather illiterate, as is evidenced by the chants and other oral mnemonics which have survived of old.
Most British Traditional Witchcraft Trads consider the Book of Shadows and all of its contents to be oathbound, a secret reserved for initiates of BTW only. Many people who have dared to publish portions of the BoS have been shunned by the BTW community and labelled 'oathbreakers'. As those who would break a solemn vow cannot be trusted to begin with, a cautionary note is usually extended to non-initiates who view any such exposé as canon or accurate, since a person who cannot be trusted to keep their word to one organization or institution certainly cannot be trusted to tell the truth.
The Book of Shadows comprises only roughly half of the lore of the British Traditional Witches. The second half is communicated and transmitted as oral lore, which is why the Book of Shadows by itself will never reveal all of the hidden truths of modern day witchcraft traditions.