Complex sentence: The book was not published during Hemingway's lifetime; it was edited from his manuscripts and notes by his fourth wife and widow, Mary Hemingway, and published in 1964, three years after Hemingway's death.
Split into simpler sentences: The book was not published during Hemingway's lifetime, but edited from his manuscripts and notes by his fourth wife and widow, Mary Hemingway. It was published in 1964, three years after Hemingway's death.