“Flanneurs,” as devotees of Flann O’Brien are called, were undoubtedly thrilled with last year’s release, from Dalkey Archive Press, of the author’s collected short fiction. Indeed, the book — including the manuscript for an unfinished novel, Slattery’s Sago Saga, a short prose typescript heavily edited by O’Brien himself, and a parody of pulp science-fiction stories which, though published under the mysterious name John Shamus O’Donnell, contains compelling evidence that O’Donnell was yet another of O’Brien’s authorial masks — seems a gold mine for scholars curious about the evolution of that wildly inventive oeuvre. Or fans whose copies of the masterpieces At Swim-Two-Birds and The Third Policeman are so well worn they are eager to devour any further discoveries from the vault of O’Brien’s forgotten prose.