A mother lies dying. Is she really sick? Will she survive? Is the writer by her side a son, a man or merely another doomed citizen? These are the questions that fuel the new novel “Tomb Song’s” rollicking, surprising and deft flurry of chapters, some as short as a paragraph, others spooling along for lush pages of artfully rendered set pieces. Mexico’s Julián Herbert, known previously mostly as a poet, is now — with this playful experiment of memoir, fiction, humor and tragedy — among the more interesting and ambitious prose stylists of our time.