With millions of copies of her “Neapolitan Novels” sold in 50 countries, Elena Ferrante is a global literary sensation. But the writer behind “My Brilliant Friend” and other sensations has chosen to shun public life, remaining all but anonymous and creating a cult of curiosity around her identity. Ferrante Fever, journeys between New York City and Ferrante’s native Italy to explore how an enigmatic artist’s visceral tales of love and friendship gained such an enthusiastic following. Ann Goldstein, Sarah McNally, Jonathan Franzen, and others weigh in on the Ferrante craze and what makes her work — and her mysterious persona — so uniquely captivating.