This, is probably one of the most definitive as well as readable accounts... Lewis & Clark: Voyage of Discovery
by Stephen E. Ambrose. Ambrose was one of our most respected historians(died last year) but also wrote a 'good' story. Remember him from Band of Brothers fame...
Also quite good: Undaunted Courage: Meriwether Lewis Thomas Jefferson and the Opening of the American West by Stephen Ambrose and finally, Lewis & Clark: The Journey of the Corps of Discovery, by Ken Burns and Dayton Duncan...