Okay, if you watched "Charmed" a tv show on TNT, then you would see this big huge book that contains information on everything magickal. Well that is what is called a "Book of Shadow".
Well in real life the "Book of Shadow" is a journal that people use to jot down; emotions, little bios about spirits and other things that related into magickal topics.
The would also place in impotant dates, spells, and rituals. The "Book of Shadow" can be of any size, many people prefer though it to be big in size. It will contain more information that way.
The "Book of Shadow" can contain what that person fears, its like a dairy or a journal. The "Book of Shadow" shouldn't be looked upon by anyone without your permission, most people curse their book, in order to keep spirits, humans, and other things out of it.