The Book of Shadows has been in the family for centuries, it is passed down to each generation of Warren witches who, in turn add spells and information to it. The Book of shadows is in just about every episode of charmed, whether they use it or not, they always check it out. The Book of Shadows is keep in the attic, that way it will not be found unless someone is sneaking around the manor. The book is protected from evil beings, and can only be removed from the manor if one of the charmed ones is taking it out. The book has lots of very useful information in it, just about every demon, warlock or evil witch the girls have come across. And when there isn’t any information about them in the BOS (as it is usually called in the show), the girls add it themselves e.g. Anton and Cole Turners human form. The book doesn’t only contain things about evil doers, some things are spells that may help the girls in their life and what is going on that precise moment in time. Something we see quite often is Grams helping the girls find a certain page they need, Grams has also written a small note to the girl on the 1st anniversary of them being witches.