Complex sentence: In September 1980, he wrote a letter to a friend, Lynda Irish, in which he stated, "I'm going nuts", and signed it "The Catcher in the Rye".
Split into simpler sentences: In September 1980, he wrote a letter to a friend, Lynda Irish, in which he stated, "I'm going nuts." He signed the letter, "The Catcher in the Rye".