In her best stories, Deborah Eisenberg plunges without preamble into the hyperactive inner world of talky, middle-aged Americans. In the witty ''Some Other, Better Otto,'' a waspish lawyer vacillates between savagely mocking his lover (''This is unbearable! I've spent the best years of my life with a man who doesn't know how to use the word 'and!''') and loathing himself for being cruel. A nervous schoolteacher entertains timid fantasies about her guide on a tour of Italy in ''Like It or Not'' until an adroit last-minute switch in perspective illuminates the terrible chasm in understanding that can yawn between new acquaintances. But the rambling title tale, Twilight of the Superheroes, about a group of young people sharing a condo with views of the World Trade Center never finds its footing. Like others who have tackled 9/11, Eisenberg hasn't figured out how to translate the enormous event into human-scale fiction.