That is one of my favorite Twain books. Life on the Mississippi comes first, following his life anecdotally from 1853-1861. Then Roughing It, covering 1861-1870 or thereabouts. Both interesting, but Roughing is more entertaining I thought.
He gives us a believe-it-or-not phenomenon that he claimed was true and not one of his many fabrications. One day in Virginia City, Nevada it had been cloudy and in the evening a hole opened in the clouds and a ray of sunlight struck the American flag on the hill. Many people there noticed it. He noted the date as July 3, 1863 making it about the same time that Pickett's Charge had failed and the South was soundly defeated at Gettysburg and Vicksburg on the Mississippi fell, both massive turning points in the war.