And somewhere in the forewards or (midwards?) included in the collection between the books by Conan Doyle it was mentioned he wrote some historical fiction novels that HE considered his best work though they've been largely forgotten compared to his more famous work.

It's true, Conan Doyle rated his adventure novels much better than his Holmes stories (he tried to kill Holmes off in The Final Problem but then had to bring him back thanks to the public backlash). I wouldn't say that The Lost World has been forgotten, though, it's had plenty of adaptations and is the Ur-Example of all those "dinosaurs/exotic civilisations live at the centre of the earth" stories.