Do spy novels come under Mystery ? They do have the same whodunit/detection thing except on a larger scale.
Is it possible that the Mystery novel (such as Christie and Doyle) got replaced by spy novels in the 80's. In my time, Alistair MacLean was a hot author (usually kids graduated from Hardy Boys to MacLean). "Fear is the Key" was the hot title. Then Sidney Sheldon etc became popular. I liked Tom Clancy and the spy theme.
After the breakup of Russia, the spy novel (and 007) moved onto North Korea which became very dull.
Nobody knows where you are,
how near or how far.