Mackenzie got this one right, as usual. A Book of Shadows is a name for a book of liturgies maintained by a formal Tradition of Wicca. It contains all of the formal teachings of the various levels of initiation in the Tradition, including the initiation (1st degree), 2nd degree, and 3rd degree elevation protocols. It will contain by-laws, charters, lists of officers in the group (the scribe, if they have one, the Handmaid, the Summoner, etc.), class handouts such as a year and a day of beginner classes, the teaching notes for teachers, a schematic of the structure of the coven (Inner and Outer courts, the licensing body used for clergy licensing of initiates, maybe passwords, etc.). Coven-specific and Trad-specific Sabbat and Esbat conventions will be covered as well.
It is one of the tragic milestones of the dumbing down of Wicca that most solitary practitioners now incorrectly believe that a "Book of Shadows" means "a magical journal." Nope. We can certainly have magical journals, and in fact, we should have them. I have several. But a Book of Shadows is, and always has been, a book of LITURGIES and of formalized documents. For a solitary to have a Book of Shadows, that person would have to have developed their own set of liturgies and written them down, thus creating a "Tradition of one." This is not beyond the ken of an advanced solitary practitioner. I wrote an article some years ago about how a solitary can write a useful BoS without wasting time and losing their minds. You can read that article here:
http://www.realwiccaninfo.com/write-your-bos
Again, it's not an insult for trained priests and priestesses to offer real information. If you as a solitary want to call your magical journal a BoS, no one can stop you. But those who respect the path understand that using educated terminology correctly identifies you as being a serious practitioner who isn't just playing witch. It takes years for a dedicated solitary to develop a BoS, and many have done just that. Whether that constitutes a BoS that actually contains something that falls within the definition of "Wicca" is another question entirely. But if your book serves YOU in a useful way, it's a good book. Call it a Rhinoceros for all I care. But if you call it a BoS and it's not liturgy, then I'm not sure what you are trying to accomplish. A BoS is liturgy, and it is usually copied from one's mother coven's High Priestess and High Priest. It is passed down from teacher to student, keeping the Craft alive as it goes.