I haven't read the particular book, but now I'm thinking of seeing if my bookstore has a copy. (We certainly have a lot of Buchan books.)

But my thought on your comments is, there was a time not so long ago, when almost *any* novel could offer that sort of ambiguity. A time before every book in the stores came with labels like Horror, Fantasy, etc, when an "adventure story" could wander upwards, downwards or sidewise into the supernatural or super-scientific.... These days we may well be slowly returning to that time as genres blur again, thanks to online book sales and related developments.