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After leaving government service, Welles became a writer and wrote books on foreign relations. His books included "The Time for Decision" (1944), "The World of Welles" (1966), and "The Last Days of the Roosevelt Administration" (1964). He also wrote a memoir, "The Memoirs of Sumner Welles" (1948). Welles was a strong advocate for international cooperation and understanding, and his books reflected his views on these topics.