Bookcasting and Messy mouth corruption
Bookcasting never fails while having this effect of corruption
The effect of corruption: Your mouth has turned into a writhing mass of prehensile tentacles; causes the PC to have difficulty speaking properly.
Hence, attempting to cast spells from memory has a very low success rate as the PC struggles to utter the formulae clearly.
Problem:
While bookcasting, the formulae surely still needs to be uttered, but the PC never fails to cast a spell from a book while having this effect of corruption.