And no, I don't mean the one politicians of all kinds are always making noises about.
What I'm talking about here is something that just about everyone gets wrong - usually because most of us are so used to the way things have worked since the Industrial Revolution that it's invisible. The changes the Industrial Revolution kicked into overdrive had already been happening, but in the space of fifty to a hundred years they went from upper class only to ubiquitous, and rearranged European society in the process. The other thing about the Industrial Revolution is that most of the changes were economic. The industry side was the means, not the end.
I'm going to start by reduction: it's easier to see the differences this way. Besides, we've all got a more or less reasonable idea of how "the economy" works nowadays. We know you earn money at a job or selling stuff and spend it on the things you need and want, hopefully in that approximate order.
The screamingly obvious first. Say goodbye to electricity, piped gas, and the car. For light, you're using either candles (expensive) or some form of oil/fat/grease light. More often than not, you're going to bed when it gets dark and getting up when it's light. Anything happening at night probably involves a full or at least three-quarter moon because that way there's a decent amount of light out. Cooking means a fire, which you've got to watch constantly and makes things miserable in summer. No cars means everything perishable is coming from close by, if you didn't grow it yourself. You're not going anywhere that's not in walking distance unless you can afford and have space for horses or there's a coach service. For most people, the effective 'range' is three to five miles for general errands, maybe ten miles if you're going somewhere and staying there, and if you've got access to good horses, fifty miles is a day's journey, more or less. Oh, and you're getting water from a well - yours if you're lucky, otherwise it's by bucket from a communal well, and the quality can be pretty iffy since there's no sewage system either. A bath means heating water bucket by bucket and pouring it into a tub as it boils. By the time you've filled the tub what you've got is - hopefully - warm water. If you're in a large family, everyone uses the tub water and the poor sod who goes last gets cold, dirty water to wash in.
Now get rid of mass-produced furniture and whatnot. Your dishes are probably wood, because for those all you have to do is cut down a tree and make it yourself. Nails are expensive because a blacksmith has to make them, individually. The old joke about granddad's axe isn't a joke any more - everything you own gets used, re-used and repaired endlessly. Your roof is probably shingle or thatch because you can do that yourself with local material, and you have to fix it constantly. Furniture-wise, you've got maybe a bed a table, and some stools or benches. Unless you're wealthy - which usually means you own a lot of land - you made it all yourself, or someone in your family did. If you're doing well, you've got some chickens and pigs, maybe even some cows or sheep. You don't usually see money at all: instead, you barter with whatever you have surplus or your skills. Money is for the wealthy. Even your taxes get paid in kind rather than in money. That's life for most people.
If you're one of the relative few who lives in the (rather small by our standards) cities, you probably still keep chickens and possibly pigs, and you might have a small garden that keeps you in vegetables. For flour, honey and such, you go to the markets. Also for cloth, since you haven't got space to grow flax or hemp or raise sheep. Cotton is rare and expensive because it's a sub-tropical crop and needs a lot of intensive labor. Mostly what you buy is some version of linen or hemp cloth or wool. There is no such thing as ready-made. You or your family make it yourself. You probably have two sets of clothing: one for everyday, and one for Sunday. Washing your clothes means dunking them in boiling water and beating the dirt out of them - soap hasn't been invented yet.
Trade means non-perishable goods - high quality fabrics, metals, gemstones, wood, dyes, spices (which are typically dried and will keep for a long time unless they get wet), lace (hand made by skilled artisans), and such. It's all expensive - your family might own one lace collar, which the lady of the house wears to church on Sundays and treats with extreme care. If you're a trader, chances are you live on the edge because travel any distance is dangerous and you could lose a fortune if your goods are stolen or the ship sinks. You probably specialize in one type of trade because of the specialized knowledge you need for it. If you're a skilled artisan, you're selling mostly to the relatively few wealthy people, or to traders, and you're probably fairly well off yourself (unless you're a blacksmith, in which case how wealthy you are depends a lot on where you're working and how specialized you are - a really good swordsmith or armorer makes a lot more than a village smith). Jobs as we understand them don't exist: most people are basically subsistence farmers or managing a little above that. People who are 'employed' are generally servants or slaves who live in their master's homes, or they're apprenticed to a merchant or artisan and learning their trade. There were a few 'open' trades - keeping an inn or tavern (which generally was done as an extra by someone who was a bit better off than his neighbors), prostitution, and hired muscle of all flavors. One of the things that kept this structure relatively static was a variety of laws that banned moving above the class you were born to.
All of this added up to a finely tuned economy that mostly excluded the majority of people. Farms and villages had their own micro-economies based on barter, with occasional luxuries like blankets being brought in by traveling peddlers and usually bartered. National economies relied heavily on how much precious metal could be mined or traded for, and on taxing trade (other taxes generally got paid in kind). Nations that were foolish enough to debase their coinage with base metals suffered inflation and lost trade. Standing armies were rare. More typically, a monarch had a small personal guard, and raised an army from the populace with promises of loot and easy victory. They only did this when they needed to - or thought they could win quickly or cheaply - because once they took people away from their normal livelihood they had to pay them. A long war could cripple a country's finances.
I'm not touching what magic does to this kind of system, because it depends on what kind of magic and what it can do. Similarly, I'm not getting specific anywhere - but pretty much all pre-Industrial cultures fit this general model, although late pre-Industrial you start having factory production and hugely uneven changes in social standing, wealth, and customs (which can and often did destabilize things). The assorted 'Daily Life in...' books offer a decent thumbnail guide to get started with. After that, I'd recommend good translations of material from, and preferably about, the period your setting is most like. Archaeological reproductions help too, as do re-enactor materials. The basic idea with this is to get the general feel of how things worked, dig out enough specifics that you can include the corroborating details (things like peeing in a bucket for your launderer tend to be more effective for Heinleining than things like the exotic carpet in Lord Sonso's manor because they illustrate the differences in everyday life).
Oh, and if you want a really good insight into war, politics (and underlying it, economics) read Machiavelli. Start with The Prince, then the Discourses to get perspectives on absolute vs republican rule in ancient and medieval times, then move to the Art of War. For double bonus points, start estimating how familiar your favorite authors are with Machiavelli. Heck, for modern war all you really need to do with Machiavelli is update the weaponry.