The term "Book of Shadows" is something that's been thrown around a lot by the New Age movement's children, and by those who want to use the term to sound more official (something that happens a great deal in many pagan and Wiccan circles). Much like the Death card from the tarot deck, popular culture like the TV series "Charmed" and bad horror movie series like "The Prophecy" have lead to a layer of misinformation being laid over just what a "Book of Shadows" is supposed to be, and what purpose it serves.
Let's back up a little bit for a wider view. Anyone who is of a spiritual or religious bent may keep a journal or diary of their religious philosophies and the feelings they get while praying or when making offerings to their diety of choice. This book is usually referred to as a ritual diary or a ritual journal, and all it does is detail what you did, when you did it and the impressions you felt when you did it. This is, at its core, what a book of shadows is... nothing more than a journal that you keep your spiritual musings in.
Now that that's been settled, let's address some of the misconceptions of what a book of shadows is supposed to be and what it's supposed to do. The popular image of a book of shadows is as a powerful book of magic written by some dark or heinous witch or warlock. The term is usually infused with fear and dread, along with healthy respect at the powers the holder of said book commands. This is mostly a misconception. Practitioners of alternative faiths (the umbrella term for those of a non-judeo-Christian bent is pagan) often create rituals or formulae for making offerings to their patron god or to a pantheon of gods. These rituals may involve speaking certain words, making important gestures, offering items as sacrifice, an altar set up in a certain way and the drawing of symbols on the wall or floors. That is the basis of religious magic, or magick if you want to be politically correct by some new age standards. You speak the right words, make the right sacrifice and intone the right prayer and the entity you are trying to invoke will grant you a boon or a wish. For many people their book of shadows is just a religious text, holding their own feelings, thoughts and prayers that they have crafted for their own spirituality.
Another misconception about these so-called books of shadows is that they are dark or evil. The word shadow refers to secret, meaning that the book is filled with secret or private knowledge. Other words to describe a book of shadows are the terms spellbook and grimoire. These words focus on the magical nature of these books, and have created the misconception that the rituals and formulae contained inside the book are somehow a dark secret, rather than just personal, spiritual practices and beliefs.
Knowing all of this, it makes it a lot easier to spot the people who really know what they're talking about when referring to their personal "Book of Shadows." Chances are that people who understand it is a religious tool meant to be a private reservoir of experience and knowledge will treat it with respect. People who threaten to use their "Book of Shadows" to cause others harm, or who ascribe the book with some inherent power of its own, may be play acting at spirituality rather than actually following it. In this case it's your knowledge of the subject, a secret in plain sight, that will give you the answer.