Hollow Earth Collection

The Smoky God: A Voyage to the Inner World by Willis George Emerson.
This is a classic fictional hollow earth adventure. The narrator, Olaf Jansen, is a Norwegian who sails with his father deep into the northern ice. There they sail over the lip of the hollow earth, and into the inner world, lit by a dim central sun. The inhabitants are an advanced race of giants who have electricity, monorails, and extremely long lifespans. The travelers return to the surface through the Antarctic, and as is usual in this genre, lose everything which could confirm their tale on the return voyage.

Symzonia: Voyage of Discovery by Adam Seaborn
This is among the first and rarest of the Hollow Earth books. In the early 19th century, John Cleves Symmes proposed that the earth was actually a set of five nested spheres, each with polar openings. He unsuccessfully lobbied Congress to fund an expedition to the poles to investigate this theory. This novel, based on Symmes' theories, was published in 1820. Some believe that Symmes was the author, but this has not been established beyond a shadow of a doubt.