I wonder if the base bonus actually works for spell casting purposes.
I have a +30% base/+20% exceptional "My Book" library talisman. When equipped and casting protection, I get the same hit on physical resistance as when it is not equipped. I understand the greater your inscription, the less physical resistance you are supposed to lose.
I have tried equipping the talisman from 0% real skill (elf, no JOAT bonus) to 70% real skill. The talisman did not lessen the impact on physical resistance caused by the protection spell at any time.
That leads me to believe the inscription bonus is either only in effect for crafting purposes, or specifically doesn't apply to the protection spell (by design or by bug).