No particular order:
Blind Descent by James M. Tabor. It is a great book about cave exploration and the race to discover the worlds deepest supercave.
A Brief History of Time by Stephen Hawking. Are you interested in the universe and how it all happened? This gives some pretty insightful answers.
From Eternity To Here by Sean Carrol. A really interesting view on the nature and concept of time and how it relates to the us and the universe. It can get a bit deep from time to time, but I found it fascinating.
Adventures Among Ants by Mark W. Moffet. It's about ants. Seriously. Ants.
The Worst Journey in the World by Apsley Cherry-Garrard. A first hand account of the ill-fated Scott expedition to the south pole in 1911-1912. Even after reading the book I cannot imagine what those men went through.
Bonus book: The Dragons of Eden by Carl Sagan. Human intelligence and how it evolved. Some really interesting stuff about the brain and how it works. A very enjoyable read.