I originally read this in the 1980s or 1990s; the author was considered up-and-coming at the time, but not really well-known.
The two main characters were a 'courier' and a wealthy student:
the 'courier' wasn't a messenger so much as an escort into the 'hidden worlds'. The hidden worlds were only accessible through a tunnel on the blasted world; this led to a world of high-tech that was treated as magic; from there, there was another tunnel to a world of real magic (by invoking sprites/imps/demons/etc).
the wealthy student was ostensibly studying the hidden worlds, her real purpose was to study the courier/escort. She was also trying to get away from a smothering bodyguard, who managed to follow them into the hidden worlds.
The denouement involved a revelation that there was a fourth world, where the demons invoked on the third actually came from, and there was a subplot involving a character trying to bring a demon into the high-tech world.