This may help: Medieval Demographics Made Easy by S. John Ross. It was written with fantasy RPGs in mind, but it's good for a rough sketch of any medieval fantasy land. {= D

Re. libraries, my sense is that buildings dedicated solely to books aren't really a thing in a medieval-flavored world. Large, wealthy establishments such as universities and churches might have a collection for the use of their members; wealthy people might have private collections; maybe you'll find a few prized volumes in the common room of a particularly educated innkeeper. But, depending on the level of technology/magic available, printing and book-binding are difficult and expensive, and the majority of the population can't read anyway. There just isn't a high demand for books.

That's just in general, though. Your world may vary! ... And I don't think you actually said medieval fantasy, but if it's not, I'm sure there are similar resources for modern fantasy and whatnot out there for the finding!

~Neshomeh

The British Library must by law have a copy of every book published in the country. I'd link to the BL website but Markdown on a phone is a pain. Basically, though, it's vast. It's got a collection of 170 MILLION books and the digital collection occupies a petabyte of storage space. So yeah, it's a chonksome building, but not impossibly so. The key is to have multiple smaller collections spread around a campus or big network of buildings. Hope that's useful =]

I generally start with climate and sources of income. Climate informs fashion, so generally you can take a climate that corresponds to a real-world society's climate and use that as fashion inspiration (do they wear furs? How much clothing is considered acceptable? How do they think about hair?). Sources of income will give you some ideas about where towns are, some minor cultural stuff, and what people are going to start wars over. So if there's, say, a super magical grove with healing powers, maybe a sick king from the neighboring country decides to yoink it. Get a little bit of war, with results depending on your themes. How does the grove feel about a war being fought over it? That can have interesting results. Income will give you things like "ok it's by the river, we've got a fisherman population. The poorer population eats the fairly untasty water plants that grow in the river, since fish is a little more expensive. Rivers let you set up windmills, meaning the farmers who are spread out across the land have reason to come to this town to grind their wheat. Maybe they all come once a year, and everyone has a fun party! The fishing town might also have, I don't know- some fancy snails in the river that can be made into really pretty dye. That means people will want to make clothing there, so we've got another industry. The clothing makers will need cotton from the farmers, or maybe flax. Since it's high-end clothing, nobles and rich people have a reason to visit the town. How do they interact with the town? Do they come around festival time, or try to avoid such peasant fripperies?". That's one town/city, depending how many other industries you add to it, and how it interacts with its neighbors. Macro stuff is often more along the lines of "is this city valuable enough for people to want to raid it?". A wealthy kingdom will only do this for a really shiny prize, but the poorer barbarian sorts have lower standards. Feel free to crib shamelessly from real historical economies and wars- we've spent thousands of years thinking up ways to make money and reasons to fight each other, so there's plenty to draw on. Good luck, and hope some of this was helpful!