First of all: sorry for my english.
I'm writing a post-apocalyptic book and I got some simple questions before I can finish shapping its world. In my fiction, the world fell apart (doesn't matter why), and people abandoned the cities, avoided them like plague. They are cursed, an image of ancient times, squeletons of metals, etc. So, a lot of knowledge is lost three centuries later. There's no fantasy here, no zombies, no aliens, etc. The new communities they founded are ruled by artisans, people who, through oral tradition (we got plenty of illiteracy in this generation of people), have the last remnant of knowledge of the world. This way, there's only one city (almost 2000-4000 population) who jelousy keep the knowledge of electricity by fixing five turbines of an eolic park, at least to light its market at night. Others are getting rich by the production of glass jewelry, exporting wood, etc.
In a matter of resources, the world has changed a little bit ('cause we warm it, for example), so you got one region of heavy rainy forest (like the northeast of my country, I'm from Argentina), and a extreme desert hundred of miles away (like the west or the south. We got a lot of biomes). There is plenty of thing to think about, like mining in the mountains of the Andes for example. I think I need some help.
My question is: Which technical knowledge could persevere that doesn't require electricity and that ain't too obvious to benefit some communities?
We all know, in a general aspect, how to cook with fire and coal, make bricks and raise a building, light it with fire or candles, harvest our food, do some metal crafting. Black gunpowder and simple gunsmithing ain't rare, but could be a thing. I need things that ain't too obvious like, I don't know, fix an aero turbine and raise electricity, use steam, etc.
Edit: Sorry, I forgot to clear out that there's no writing information of the past (at least not understandable). Doesn't matter the details, but there's only oral tradition, as I mention before.