The Friends You Meet: O'Brien, Robert C. The Silver Crown. London: Victor Gollancz, 1973.

Not at all sure this is sf. Ellen wakes on her birthday to find a crown on her pillow, She takes it to a park and when she comes home her family is dead and the house burned down The policeman who tries to help her is killed. She sets off to walk to Kentucky to find her aunt Sarah. She meets Otto who lives with a lady in the woods and wrecks trucks. This section is a classic braacelet fantasy in which Ellen collects assistance and we learn about Ellen.

When Otto is captured she goes to the dark castle and finds everyone conrolled by the black paths, made of malignite (which is a classic fantasy conjuration), and working out how to terrorise the world, with no particular goal in mind. The best scene of the lot is the one in which she sits in on a class on terrorism. I don't think O'Brien meant this one to be the scene I would have remembered for two decades.

Eventually Ellen rescues her crown from where she had left it and puts it on, and the silver crown turns out to run the Heironymous machine which had enslaved everyone in the castle, including the "King". Ellen destroys the machine.

The accoutrements of sf are there: the Heironymous machine spawns an industrial complex to produce more of itself, and to spread itself through the world, but the structures and themes are of fantasy -- destiny, mastery, control and free will.