As for when the books restock: I'm pretty sure it doesn't have anything to do with actual time passing. It seems as you progress and achieve things, the merchants all over, including the Teller, will update their stock. Like let's say discovering the mining village, or like, when you first get to Hiberheim or something. It's after milestones of that type that I think it's worth seeing about a restock. Let me just advise this: Whenever you're going into something that you know is going to count as a big step in your progress, might be worth going back to check if you've done some stuff already as well.
