Most books are about how to get to the point of casting your own spells. There is a good reason for that.
Most people, if given the grimoire/book of shadows/spellbook/notebook/whatever from an experienced magician/witch/mage/whatever wouldn't be able to make heads or tails of it, or even have the spells work.
This is because as you grow in strength and knowledge, you learn instinctively. The pieces you pick up as you go are individual experiences, things that books and other's notes Cannot explain. Things that we do not Have the language to communicate.
That is a good reason to start from scratch, if anybody Needed one.