A Book of Shadows is a lot more than a recipe book of spells. It is more of a workbook or magickal diary which corresponds to, and records, your journey and experiences. It evolves as you do.
Yes, witches use their Books of Shadows to write down their spells, incantations, chants and rituals. But they also include descriptions of stones and herbs, the meanings of runes, records of their dreams and trance journeys, poems, reactions to and effects of spells and rituals, history and lore, meditations, definitions, blessings, recipes for medicinal herbal remedies, planet and color correspondences, moon cycles, rules governing magick, etc.
In other words, anything they feel is important to them and that they want to save for future reference.
Some Books of Shadows are handed down from one witch to another, usually during an initiation ritual, within a coven or family. But most are composed by each individual practitioner. It's a good idea, even if you belong to a coven which shares a common Book of Shadows, to keep a personal one of your own. It will have more meaning to you and will be more helpful to you, as well.
Until recently, a Book of Shadows was always handwritten (sometimes with colored or scented inks which corresponded to the magicks being written about) on onion skin or parchment paper. It is a lot more common nowadays to see them written in spiral notebooks, loose-leaf binders, or even on floppy discs or up on webpages.
To create your own Book of Shadows, all you need is a blank book to start. It can be a notebook or a bound book with blank pages, such as a diary or a record book. Simply write in this book any information, spells, rituals, invocations, etc., that you have either composed or found elsewhere and would like to preserve.
In my opinion, for those who are not yet out of the "broom closet," so to speak, or have people in their lives who do not understand or approve of practicing the Craft, keeping your Book of Shadows online is an excellent way to keep it from being discovered by those whom you wish to keep it a secret from. You can store your entries on webpages to which only you have the URL, in free online file storage space, or even email them to yourself and save them to a folder in your web-based email account, such as Yahoo! or WebBox. In fact, Webbox offers 20mb of file storage space along with their free email accounts. But, if you do decide to keep your Book of Shadows online, it's a very good idea to keep more than one copy in more than one place, just as a precaution against loss due to server error or breakdown.
Another thing to note: Scott Cunningham said (in Wicca: A Guide For the Solitary Practitioner) Don't believe the stories in most other Wiccan books that one single Book of Shadows has been handed down from antiquity, for each sect of Wicca seems to claim that their own is the original, and they're all different.
He's right. This is strictly folklore. There are very few written accounts of magickal practices which have survived through the ages, and no single one is "the" one. They are as individual as the practitioners who penned them. In fact, I would be surprised if most practicing witches during the Burning Times (and even before) wrote down anything at all. First of all, a lot of them could neither read nor write and they passed down information by word of mouth from mother to daughter, coven members to initiates, etc. For another thing, it would have been like signing their own death warrant if it were ever to fall into the wrong hands.
Remember, too, that all Books of Shadows, are a record of and suggestions for magickal practices. They are not "holy writ" nor carved in stone. Never feel tied down to or locked in by them. As I said earlier, they should evolve as you and your magick do. It's perfectly all right to add to and subtract from your Book of Shadows as you see fit to do.