The Old Phoenix And After Such Knowledge

There is a rather obvious parallel here although it is not necessarily immediately apparent because the differences are even greater. However -

Poul Anderson's Operation Chaos is about magic and has a sequel, Operation Luna. Both works have been collected in a single volume entitled Operation Otherworld. They are connected to two other novels and to two short stories. The common theme is parallel universes and there is an inter-universal inn called the Old Phoenix.

James Blish's Black Easter is about magic and has a sequel, The Day After Judgement. Both works have been collected in a single volume entitled either Black Easter and The Day After Judgement or The Devil's Day. Blish suggested that an appropriate joint title would be Faust Aleph-Null which is the sub-title of Black Easter. The two works, considered as a unit, are the second volume of a trilogy entitled After Such Knowledge whose common theme is the question whether the desire for secular knowledge is evil.

In Black Easter and The Day After Judgement and in Operation Chaos, magic works, Hell is visited and the Devil speaks. Blish's Hell is Dantean and subterranean whereas Anderson's is entropic and extra-cosmic. Blish's magicians operate in secret whereas, in Operation Otherworld, magic has become an everyday technology on a parallel Earth. The differences in detail are greater despite the structural parallels.