"Day by Night" (published 1980) is a young adult story by Tanith Lee. Set in a world which does not revolve, the heroine lives in the side in endless day, the hero on the side in endless night. To each the other is a fantasy. Yet their interleaved stories mirror each other, and their fates intertwine.

Rich high fantasy, full of Tanith Lee's decadence and magic. This is science fiction but the technology is advanced unto magic.