If "illustrative essay" means one that gives a birds-eye view of a major cohesive point that determines the novel's texture, then one suggestion might be to discuss the structure of the novel. Frank O'Connor writes in the literary tradition of realism; his characters and the events in his stories are everyday people engaging in everyday occurrences that show how his characters function on the most essential levels of life, such as a Catholic lad's ritual first confession. Contrast this to characters who fight dragons, find chests of gold, climb impossible mountains or even fight in world wars. O'Connor often contrasts the youthful perspective on life's common occurrences to the adult perspective(s), as he does in "First Confession." In addition to which, he casts "First Confession" in a subjective point of view by making young Jackie the narrator, which allows O'Connor a vehicle for humor, irony and unusual observations (a nine-year-old's observations are sometimes unusual such as when perched on an armrest in a confessional). Your essay might discuss the instances, affects and importance of these structural points, thus producing a discussion illustrative of "First Confession."