A Book of Shadows is essentially a workbook which contains rituals or ritual structures, spells, thoughts, musings, gathered knowledge, invocations and observations relating to the magickal workings of a witch.
Some people keep their BoS in glorious notebooks with fantastic covers.
Others have their BoS in a looseleaf binder, or bits of paper slung together in a folder, or a school notebook. Some keep them only on their computer. I personally believe that a BoS should be written in your own hand and if you want to put it into your computer later, that's fine. I like the process of hand writing my BoS because it connects me to the words and images much more. What you do after that is up to you.