Okay, so I don't think I fully understand spell tiers.
I understand at the start of the game I have spell blah blah I and spell yadda yadda I. But then my books run out. Do my spells automatically upgrade after a certain civ level? Do I have to find new books? And at what civ level should I start searching for book to make tier II spells (and so on).
Basically, how do I get the next-tier spells?