Here's another novel that I read in High School that I'd love to track down - it's in English, and must have been written in 1993 or earlier. I know that I got it in my school library, so it is almost certainly a Young Adult book.
As far as I can recall, it was a stand alone book, not part of a series.
I seem to remember that the cover showed a young person or two, standing in a field of grass with two giant ferrets or weasels. I think they were pets or companions of the female lead of the book, although other than them being on the cover, I don't remember anything about them from the text at all.
This was a post-apocalyptic book, there were still a handful of broken & overgrown paved roads around, but, the ‘modern’ world was basically forgotten. There were lots of mutated animals and beings around.
I don’t remember anything about the main plot, but I remember a scene where some fishmen (maybe frog-people?) came out of a river and attacked a tribe (warband?) of silver horsemen (maybe they were mounted on deer?). Specific mention was made of the fishmen’s use of barbed spears, with the barbs breaking off in the wounds of anyone they stabbed. The silver people (maybe they were silver-clad, not literally silver – my memory of some details is very sketchy here) were silent, and after the warriors were killed, the silver women and children stood silently and let the fishmen kill them, too.
That's the scene that's stuck with me the most - I remember thinking it was utterly horrifying the way the noncombatants just stood there and let themselves be killed by the fishmen, presumably for food.
The main characters eventually discovered a bunker or compound that had an ancient man in it – he called himself the “Last of the Children”, he had been the youngest child of a group of scientists manning the computers in this compound. I don’t think his exact age was mentioned, but I think he was hinted to be several hundred years old, the last remaining bits of super-science in the compound had kept him alive.
I think the bunker/compound collapsed or blew up a the end of the book, but I don't clearly remember that part, either.