Thicken skin says that "[y]our skin or natural armor thickens and spreads across your body, providing a +1 enhancement bonus to your Armor Class."
The text for enhancement bonuses says:
An enhancement bonus represents an increase in the sturdiness and/or effectiveness of armor or natural armor, or the effectiveness of a weapon, or a general bonus to an ability score. Multiple enhancement bonuses on the same object (in the case of armor and weapons), creature (in the case of natural armor), or ability score do not stack. Only the highest enhancement bonus applies. Since enhancement bonuses to armor or natural armor effectively increase the armor or natural armor's bonus to AC, they don't apply against touch attacks.
Thicken skin provides a +1 enhancement bonus to AC; it does
not provide an enhancement bonus to armor, a shield, or natural armor. It's just a bonus to your AC. So, my question is, does it stack with armor that has an enhancement bonus? (For instance, if I cast
thicken skin and am wearing +1 plate mail, do I have +9 AC or +10?)