September on Nantucket is the finest month. The beaches empty, the coffee lines shrink, and Main St. parking spaces reappear as if by magic. It's the near-death experience of summer, just before the island folds its big, noisy tent, begins closing down all the nonessentials. It's a time for stowing away the glitter and the folderol, when caretakers toss mothballs and rat powder into the darkest corners of all those little shuttered wharf boutiques. It's the time for hauling tourist boats ad launching scallop boats.