A "BOOK OF SHADOWS" is a personal "Book of Spells" that you write for yourself. Different people have different books, usually referred to as a BoS. Technically, another term would be "grimoire" but people don't really use that in general conversation. Some witches prefer to call theirs a Book of Light, or any number of other terms. It's not set in concrete, but the most common term for a witch's book is a Book of Shadows.