Why does magic go into books? If I remember correctly, the first things to go to writing were accounting records... The rest was pictures, drawings, things that feel closer to magic.
Writing seems a tool of empire, and I have the impression magic is more peripheral, it's pushed into the corners. Could the romans have gotten rid of the druids and their political influence, splintering their organizations, pushing them into the periphery?
I imagine magic to be a kind of gift, that one discovers and doesn't understand. It probably requires a master, someone who discovers the gift in you and guides you towards control over it. Or else, the gift is given to everyone and the master awakens it and teaches its processes.
Maybe our medieval vision of books as very expensive items made by very skilled people, is tainting our view of magic. Books must certainly hold magic, they are so expensive and you need the reading skill to use them.
I prefer magic to be more on the side of WIS than INT, magicians closer to artists than to engineers.
Sixty years ago, my grand-mother's mother in law died at 80, she had put fuel in the fire in order to start it to prepare breakfast. A neighbour woman told my grand-mother that death by fire was common to people practicing magic and that her mother in law had a grimoire. My grand-mother simply thought the neighbour was badmouthing the dead woman and never searched for the grimoire.
So I might be wrong, and magic gladly rides literacy.