Madeline King and Grace Pancik are best friends and the envy of Nantucket for their perfect marriages, their beautiful kids, their Saturday night double dates with their devoted husbands. But this summer, something's changed, and if there's anything Nantucket likes better than cocktails on the beach at sunset, it's a good rumor. And rumor has it . . . that Madeline, a novelist, is battling writer's block, with a deadline looming, bills piling up,...