A Book of Shadows need not be an actual book! Mine is a collection of notebooks gathered for many years. Think of a BoS as a Journal, a Diary, with a difference. The difference is that you might add all sorts of "Scraps and Shavings", little pictures,photographs, anything that is of interest to you now, that you may look on in later years and say,"Oh,I remember that!".
But mostly a BoS is a journal of the magic in your life; and the many things you will learn about it throughout a lifetime.