From the introduction:
The origin of magic is lost in the sands of time, but its power remains, jealously guarded and passed down from master to student over the years. Magical knowledge is often stored in writing: books and grimoires, scrolls and tablets. All magicians are taught to read magic’s ancient language, and most keep a spell book or collection of writings, storing those spells discovered, taught, or stolen. Illiterate magicians are limited to spells memorized.
This is the second volume in a three volume set for the core rules.