In Book 3, Chapter 1, Fielding intrudes into the narrative of his novel once again. This time, he's here to lavish ironic praise on works of biography as the only real sources of truth in literature.
Notwithstanding the preference which may be vulgarly given to the authority of those romance writers who entitle their books “the History of England, the History of France, of Spain, &c.,” it is most certain that truth is to be found only in the works of those who celebrate the lives of great men, and are commonly called biographer.
The verbal irony of this passage stems from the tension between Fielding's statement and the obvious status of his own novel as a work of fiction. If truth is "only" to be found in biography, then the reader has spent hundreds of pages reading Joseph Andrews for nothing—despite Fielding's profession, throughout the book, that his own novel contains valuable truths for the reader to ascertain. Fielding obviously believes in his own work and in the capacity of fiction to contain valuable pieces of moral advice, and thus the statement above emerges as a prime example of his playful, self-deprecating, and ironic digressions into the nature of his craft.