It seemed to me that the book divided itself into 3 themes...the first one seems personal, the second seems to validate his value as a person, and the last is to promote his religious and political ideas.
Franklin's tone starts out as humble and he claims to write only so that his own life may be an example for his son and others of how one can live well and how one can get through hardships. For example, Franklin mentions his errors that he would have corrected if he had the chance to relive his life.
Later, Franklin seems to promote himself as an American legend who is responsible for many improvements in American life. He discusses most of his common inventions that we have always associated with him--the discovery of electricity in lightning, the invention of the fire brigade, the work as Postmaster General, the funding of a hospital, the organization of a street-sweeping force, and many others.
Franklin spends much time towards the end recounting the events of the French and Indian War as he took part in them, as though to ensure the reader that he took part in all manner of services towards his country. He also gets more into the reasons and background that laid the groundwork for what could be a thorough discussion of the Revolution and Franklin's role in it.