Why is Dunny unhappy with the school letter? How does this relate to the title of the book?

The entire narrative is a letter to the current headmaster of his school, to complain about a school newspaper article detailing his retirement. Entitled "FAREWELL TO THE CORK," the article implied that Dunstan had never had much of a life outside of the school and was only a reliable but boring fixture there (13).

He takes great offense to the implication that his life has lacked in meaning and that he possesses little personal wisdom to pass along to his students. Finally, he is offended by the article's suggestion that he has a "bee in his bonnet about myth," when in fact myth is the prevailing interest of his life in one way or another (14). Thus, he has written this letter to set the record straight, to explain that his life has been full of adventure and remarkable occurrences. The letter implies that he has been Fifth Business all his life, a mere spectator in life enabling other more important people.