Is this a plot hole (spoiler for part 2)

This has bothered me since I saw the musical. How is Nessa able to read the Grimmerie AT ALL? So much goes into establishing that nobody else except maybe Morrible has been able to use it? It seems like a plot hole that any person can just grab it and try and seemingly cast spells, even if they mispronounce the words.

I’d love to hear your thoughts!