A Book of Shadows is the personal journal of the spiritual explorations of the owner. ;It is a record of personal experiences, insights, and magick. ;In it might be written copies of rituals done, spells performed, notes on dreams or visions, inspirational poems, words of wisdom, images and pictures, or anything else of spiritual note that contributes to the spiritual growth of the creator.
Many established Wiccan Traditions will have a fairly well defined core Book of Shadows, which must by copied by hand as part of the training of an initiate. ;However, given the popularity of technology in the Pagan community, these days many Books are kept entirely in digital format.
A Book of Shadows does not have to be embossed leather with a gold clasp. ; In fact, it gets such heavy usage that it should ideally be chosen for the ease with which you can add and change around pages. ;I strongly advocate using 3 ring binders... you can add full pages from your printer, shuffle the contents, and add 3 ring "zippered pouches" to hold all of the trinkets and little stuff that would fall out of a "blank book" diary.
There is speculation on why it is called a Book of Shadows (most commonly abbreviated to BoS). The most common explanation is that during the Burning Times (the period of intense witch hunts that occurred in Europe during the mid-fifteenth through eighteenth centuries), these types of magical notebooks were dangerous to posses, and were therefore kept in the "shadows".
Actually, in the Burning Times, they probably weren't written at all, given the high illiteracy rate and the mere fact that being found with such an item would be too dangerous. ;But even when we aren't being actively hunted down and killed, it's still best to keep such a personal work of spiritual progress to yourself, or entrusted to only family and close friends (such as found within a coven).
~Flame RavenHawk