Why the Book of Shadows in the Wizard Kit is Blank
Magic comes from a very large number of traditions, with a large number of skill levels and magical knowledge allowances. Not all spells in a tradition were given to a new magic user. In much of history magic needed to be secret, often leading to witches and wizards practicing in solitary.
In all of these traditions you started with a blank book. You will need to record your magical knowledge as you learn it. When you learn a new spell, record it. When you come across another who has a book of shadows you can ask them if you would be allowed to copy some of their spells, if they agree they will share some of their magic with you.
A book of shadows is a personal magical journal and personal spell book, it needs to start blank.
You can own 100 other printed spell books, but only one book will hold your personal set of chosen spells and magical knowledge, and that's your personal Book of Shadows (unless you're like me and own a dozen of these self written books).