A BoS (short for Book of Shadows) is basically a notebook, journal, binder, or anything that contains information on the craft, your experiences, spells, pictures, and anything else you'd like to add to it. Make your BoS your own and be creative with it. Writing information in it is also an easy way to go back to it and share it with others if you choose too (just make sure to include sources).
My BOS is a binder so that I can add to it and it will expand without problems. I also keep a copy on my computer and a zip drive so I can print the pages off if something happens to any in my book and in case my computer crashes.
You can add anything you want into your BOS. You can add a poem you wrote to recipes for a holiday dinner. You can add spells and rituals, herb information, crystal information, candle information and the colors and properties of these. You can journal in your BOS, it is yours to add what you want. It is your religous diary to use for future information.