There are as many different answers as there are end of the world novels.
If we're just wanking along, what to me seems most likely is a 'Fall of Rome' deal where things gradually get worse but no one fully realizes it because it happens so slowly.
One doesn't know they're living in the ruins of an empire, it's just the way things are.
Still, would knowledge go away today? Interesting to consider what would be the minimum sensible size for a modern society? For example perhaps acquiring raw materials like iron would be a huge problem?
IIRC, medieval farmers in Gaul had crop yields a fraction of their Roman ancestors farming the same land. No soil depletion - they'd forgotten how to farm.
This seems silly - it's _food_. It would be like people in 21st century forgetting how to refrigerate their food. 'The meat goes bad so quickly now, I wonder why?'
But it happened.
"minimum sensible size for a modern society?"
The number 10,000 is in my head, but I don't know if that's an actual real number of something a scifi writer made up.
"cquiring raw materials like iron "
Well .. every guy at the mine face digging out ore is supported by .. well a whole lotta people. Someone makes the tools, generates electricity, makes lightbulbs, and pumps, and motors, and food, and builds and maintains the railroad getting the ore to the smelter. The guy at the smelter needs lights, and electricity, and food, and tools.
Border defense! If you've got civilization there are going to be people outside of it who want to rip off what you've got. Better make sure the army is fed, and supplied.
"My hope would be to find some books on farming..."
The local library was looted when the Empire withdrew from the outer provinces.
Big deal! You're a farmer speaking medieval French: the books are in Latin.
And you can't read anyway - no schools.
And even if there were schools you couldn't go. As soon as you could walk Dad needed you for a farm hand. For some reason we don't get the crop yields Gram'pa did and we gotta work can see to can't see just to get barely enough food to survive.
After that the roads are going to be torn up by weather and no maintenance and then riding a bike may or may not be such a good choice.