Instead of spellbooks having 4 read charges, they store “400% memory” - enough magical power to give a reader full knowledge of the spell 4 times over, which is functionally identical to how many times they can be reread now but more granular. Reading a book to restore memory of a partially forgotten spell only consumes the missing fraction of memory required to bring you back up to 100% retention.

Example: The hero reads a spellbook of knock, bringing themselves to 100% memory and the spellbook down to 300% memory. 10000 turns later, the hero, now with 50% memory, rereads the book. This restores their memory to 100% but the spellbook is only decreased to 250%.

This would allow events that degrade spellbooks (reading it and polymorphing it) to degrade it by a randomized degree instead of a flat, predictable 1/4 of its use every time.