The power that a book of shadows has is that it is often the journey of the witch/practitioner that wrote it.
A book of shadows is a journal (lack of a better word) of a witch or coven. It contains the knowledge and workings of the writer. It is where one can recorder spells, correspondences, basics, beliefs, rituals, dreams, etc. There is no one way to make one, everyone does it differently. It has different names for different practitioners. It can be a bound book, several notebooks, a three ring binder, or a computer program (or app on a smartphone).