I read this book too many years ago to give a good summary so I am stealing from Amazon:
"A biography of Meriwether Lewis that relies heavily on the journals of both Lewis and Clark, this book is also backed up by the author's personal travels along Lewis and Clark's route to the Pacific. Ambrose is not content to simply chronicle the events of the "Corps of Discovery" as the explorers called their ventures. He often pauses to assess the military leadership of Lewis and Clark, how they negotiated with various native peoples and what they reported to Jefferson. Though the expedition failed to find Jefferson's hoped for water route to the Pacific, it fired interest among fur traders and other Americans, changing the face of the West forever."
I thoroughly enjoyed this book! The story is captivating and truly an adventure. I am a big fan of Stephen Ambrose.