I have a couple of reference books for some of my go-to topics, but it's hard to extract my research from my writing process, you know? It's sort of... I figure out what I want to have, and then I figure out how that makes sense. That process is actually really fun for me, and I think a lot of cool world-building can happen in the process.
Let's say you want a 1700ish setting. The set thing is the clothes. You need the structure that can make that fashion, and everything else if up to you. The thing is, you aren't stuck in the 1700's exactly! Scientific progress isn't a straight line, it's driven by need and what a culture values, along with chance. A culture with magic might use that magic to start using electricity in the stone age, and they might have TV's without ever discovering it because they can power things with magic.
So why did fashion look like that in the 1700s? It's usually defined by:
-What textile skills were possible
-Attitudes towards beauty and gender (most 1910's dresses emphasize an hourglass figure, while the 1920's was all about rebelling against that and emphasizing a squared, boyish figure)
-Class. There are times when fashion was all about showing your wealth as dramatically as possible, times where it was chic to be "working class but rich", and times where simplicity was valued.
I think the most stereotypical image for 1700s fashion is French rococo, a fashion that was all about opulence. That style didn't survive the revolution, as the opulence of the ruling class became a symbol of the people it exploited. So, to me, the main thing I expect from a world when I see those dresses is heavy class stratification and great pride in wealth. This isn't a time period where billionaires awkwardly kick their inheritance under the rug and claim to be self made, it's a time when the rarest gems and the most heart to sew fabrics are vogue.
So what keeps that structure in place? Magic can put a lot of power in the hands of the general populace, so what would restrict that power. What's the role of the church? Who could get educated in the 1700s?
The 1700s also had a lot of colonial activity, and a huge way to show wealth was rare imports. If you don't want to get into the more nasty parts of what made luxury possible and what that means for you world, it means figuring out what would take those roles and what that meant.
If you're writing royal drama, I think it's also interesting to get into exactly what nobles did in this culture. Plenty of stories go with "have power and do nebulous paperwork", but it can be fun to get into... what a duke actually did.
This is a fascinating thing about the role of eccentric guests in historical fiction.
If you have the time, reading fiction from the actual time period can also be really interesting and give you a better idea of what the concerns of the rich and famous would actually have been, especially since most old novels were written by those with plenty of free-time and education.
Note: This is really in the weeds shit, and not the place you need to go, but it is how I do research.