The Vera family had the Book of Shadows, the Elders kept the Book of Elders, and even Abby had her mom's potion book. It is clearly commonplace for witches, along with non-western magic practitioners, to create a tome in order to pass on their wisdom. Most witches don't even seem to fight demons and have still created these book. The Charmed Ones, however, specifically come into existence/are chosen when the world needs them which indicates that each group of sisters had battles of their own so it would make sense for them to create a book to passed on to the next group of charmed ones, especially since some charmed one fought the Perfecti & multiple groups tried to find a way to overcome the Power of 3's cycle of destruction. Also, I assume the lore about multiple Charmed Ones was thought of in Season 2 but in hind's sight it looks like the Elders were hiding it from the Vera-Vaughns.