Lost Races:
Lost Horizon by James Hilton
The Lost World by Arthur Conan Doyle
The Devil's Guard by Talbot Mundy
http://www.violetbooks.com/gal-lostrace.html
Hollow Earth:
Journey to the Center of the Earth by Jules Verne (they only go down less than 100 miles)
The Narrative of Arthur Gordon Pym of Nantucket by Edgar Allen Poe
A Strange Manuscript Found in a Copper Cylinder by James De Mille
Abduction by Robin Cook