Ah yes. She first talks to the flames first, and then from the books learns it’s forbidden magic. My interpretation was “oh, here are books that explain the thing I was learning on my own” and considering that Keigetsu gets proficient enough at it to body swap and contact Reirin at will, I just assumed her dad’s books were more of the “Black Magic for Dummies” variety. But it’s totally possible the notes were “Earth/Fire/Water/Air magic = good. Black magic = bad.” and she learned everything on her own.