A recent best seller was Into the Wild. The same author wrote another book about climbing mountains in Washington state and then climbing Mt. Everest.
There is a book about John Wesley Powell who was a one armed ex-civil war officer. He was the first person to explore and traverse the entire Grand Canyon. Excellent book.
Colter's Hell is about John Colter's exploration of Yellowstone. He was discharged by L&C on their return down the Missouri so he could return to the mountain to trap and explore and excape civilization. He has some interesting encounters with the Indians.
Another good book is about the connecting of America's railroads. Written by same man who wrote Undaunted Courage. He writes a lot of good stuff.
If you enjoyed Lewis and Clark, the Univ of Nebraska publishes a seven volume complete journal of their exploration. The footnotes in the journals were the most interesting parts, because they often explain mistakes L&C made about geography, mapping, plants and animals.
I also enjoy reading about explorations. What are some classics other readers have discovered?