This book I have appears to be an adventure novel .I haven't read They Found Atlantis.
The ones I do remember
The Forbidden Territory
The Devil Rides Out
They Used Dark Forces
To The Devil a Daughter
Gateway to Hell.
I've read others but can't remember.
This was quite some time ago... 1980? I was a student. You never see Westerns on store shelves today either, but there was huge shelf space devoted just to Westerns in libraries back then... Ah! Yes, the old days!!!I've never seen any of his other books on store shelves.
I may have seen both of those. bot, Im not sure .The Devil Rides Out (1968) and To The Devil, a Daughter (1976) were filmed, both with Christopher Lee, both from Hammer. Don't recall seeing the latter but the former is enjoyable, with a screenplay by Richard Matheson. Pops up in the U.S. on TMC every so often.