The History of the Adventures of Joseph Andrews and of his friend Mr. Abraham Adams is a mouthful. We think it's pretty significant that the full title includes both Joseph and his right-hand man, giving them nearly equal billing.
Joseph is definitely the star of the show, but Parson Adams is Fielding's personal addition to the Pamela franchise. While Joseph himself is straight-up satire of Pamela Andrews, Adams adds his own particular brand of tomfoolery to the trope of the bumbling parson. Plus, Fielding pretty much promises to give us a new species of writing in the Preface. We're thinking that the "History" part of the title shows us which direction that writing will take.