Gerald Gardner coined the usage of the term book of shadows after noticing a tenebrancy book of the same name advertised in a occult newsletter. His definition of the term was the coven’s book of rituals that initiates were to copy by hand for themselves (Per report from Doreen Valiente). When Cunningham was writing his books, decades ago (he has been dead a while now), books of shadows were still only a Wiccan thing. Now most people who use the term are not wiccan and do not use it for coven rituals—but instead as a magical journal, solitary rituals and as a spell book. Cunningham’s work is about solitary witchcraft and usually has some references to gods and goddesses but you can ignore that if you want as a secular witch because Cunningham wrote his books for solitaries making their own paths.