One might naïvely suppose that this would be an easy distinction, for
both books and games. Fantasy has dragons; science fiction has
spaceships. But there is a set of ideas, loosely correlated with the
SF/fantasy divide, which to my mind make a greater difference to the
feel of a story than do trappings like those.
The mystery story when done well is an extreme form of the story
problem, which one can enjoy both on the level of normal fiction
with characters and plot and setting and so on and on the level of
working out whodunnit before the author reveals the solution.
John Dallman invented the term Occult Secret Service to describe the
Laundry novels of Charles Stross (from 2001) and the Broom
Cupboard novels of David Devereux (from 2008). It's become popular in
other fiction and in role-playing games. Is it useful to analyse it