Complex sentence: Lincoln's life story is additionally reported by Lincoln's law partner William H. Herndon (Lincoln's biographer) that on his way home to Illinois one time between sessions of Congress in 1848 the boat Lincoln was on became stranded on a sandbar.
Split into simpler sentences: Lincoln's life story is additionally reported by Lincoln's law partner and biogapher, William H. Herndon. Herndon reports that on his way home to Illinois one time between sessions of Congress in 1848 the boat Lincoln was on became stranded on a sandbar.