Peter Matthiessen died of leukemia April 5, three days before publication of his curiously unfulfilling final novel. A complex man whose 86 years encompassed espionage, writing, naturalism and meditation, Mr. Matthiessen focused on big themes such as ecology, spirituality and identity. Emphatically not a writer of “entertainments,” he is the only person to have won the National Book Award in both fiction (“Shadow Country,” 2008) and nonfiction (“The Snow Leopard,” 1979).