A Book of Shadows is like a witches journal. We write our spells/rituals/potions in it, we write effects of a spell/ritual/potion, and we write what we experienced on our path. It is basically a "diary," where you put in your thoughts and feelings, and the date you wrote it.
The Book of Shadows is not supposed to be shown to others, it is secret. Like i said, it is like a diary. Your Book of Shadows should be personalized--after all, it is yours. You can use anything as a Book of Shadows; a composition book, a hard cover book, a journal, the computer, etc., as long as its kept as a secret and it's personalized, unless you like it plain.
I use a composition book, then I have personalized it with a book cover (or Book Sox). You can decorate it however you want, it is up to you.
If you run out of space in your Book of Shadows, you can get another one and "bind" them together, with a book cover as I have did. What you do is that you put your first book on top, and your second one on the bottom. You take the first cover of Book 1 and the back cover of Book 2 and put the Book Sox on that.
But basically, your Book of Shadows is a secret diary/journal, whatever you like to call it.