The longest by far of Tanith Lee's Flat Earth books; also a direct continuation from Delusion's Master (the third book) -- per the introduction, Delusion's Master/Delirium's Mistress likely would have been one really, really long book were it not for a) the exigencies of bookbinding and b) the fact that Lee got distracted by another project between the two books.

Anyway.

Delusion's Master was the story of Chuz, third of the Lords of Darkness, and the tricks he played on Azhrarn, Night's Master, and the fate that was meted out to him. Delirium's Mistress is the story of Soveh/Sovaz/Azhriaz, Azhrarn's daughter (by Dunizel, Azhrarn's mortal love, who perished as a result of Chuz' actions at the end of the previous book; needless to say, there were Consequences for such actions). At first, her father sends her back up from the Demon realms below to the Flat Earth to rule as a goddess (and, not incidentally, inculcate a strong distrust of the Gods amongst the dwellers on the Flat Earth). The Gods themselves, from their dwelling places far from mortal ken, do eventually become aware of, and take exception to, Azhriaz' actions and send three fiery angels to convey their displeasure.

Needless to say, Azhriaz has her own thoughts on the matter ...

Again, the world and the language is in equal parts gorgeous and cruel, and ultimately the power of Love (and the power of Death) is not to be sneered at.