The First Men in the Moon (Dell Mapback No. 201)

Set in England at the beginning of the 20th century, average industrialist Bedford finds himself entwined in the machinations of Cavor, an eccentric genius who has developed Cavorite, a substance that negates the pull of gravity. The two men construct a vessel which hurls them to the moon. Cavor hopes to discover a utopian society he imagines living on the planet, while Bedford is purely interested in the monetary gain the trip represents (after all, everyone knows there's gold on the moon). Once they arrive, they stumble upon the world of the Selenites, insect-like, biologically engineered aliens living beneath the surface of the moon in dark, cavernous, technologically-astounding cities.