Project Gutenberg takes us back to the era of Hugo Gernsback with a Lost World novel and a space opera novelette.
Two Thousand Miles Below (A Four-Part Novel) by Charles Willard Diffin (Astounding Stories June, September, November 1932, January 1933)
"Rawson learns to his cost that the life-spark of a fabled race glows in the black heart of a dead, Western volcano."
Slaves of Mercury (A Complete Novelette) by Nat Schachner (Astounding Stories, September 1932)
"Hilary returns to find alien diskoids in Earth's stratosphere, and out-world lords patrolling her cities."