The Dust of Ages: Hugh Scott, Why Weeps the Brogan (London; Walker Books, 1989).
Winner of the Children's Whitbread in 1989.
This is a very strange book: the protagonists are two children, but they don't know enough to know they are children. They live in a place dusty and gloomy with signs such as ARCHEOLOGY and EGYPTOLOGY to tell them where they are. They dress in skins and use spears to kill the spiders until they discover the EXTINGUISHER, and they feed and fear the Brogan.
A sign on the wall tells them that it is four years and 81 days since the hostilities started.
At the end of the book the two work their way to the outside world, and the older of them, the girl, recovers some of the memory she lost on the traumatic day when her mother yelled at her to get into the museum and to stay there, never to come out, before she dragged herself in with crippled legs.
None of this quite explains why it's so good. I think it's because Scott stays rigorously with the ignorance of the girl (Saxon) and because he eschewed first person narrative he combines this with our puzzled observation. I had a good idea what was going on, but Scott made me occupy the world view of an explorer. Awfully good: get it if you can.