History of Joseph Smith by His Mother is a biography of Joseph Smith, founder of the Latter Day Saint movement, according to his mother, Lucy Mack Smith. It was originally titled Biographical Sketches of Joseph Smith, the Prophet, and His Progenitors for Many Generations and was published by Orson Pratt in Liverpool in 1853.
Lucy Mack Smith thought her son was the Prophet. 
If the book was originally titled "Biographical Sketches of Joseph Smith, the Prophet...." and was written by his mother, it logically follows that his mother thought he was the Prophet. Especially in that time period and that subject, you weren't going to be ironic or cute with a title. You would be serious about it.