I'd say that the "from" version suggests he sends his fiction to some place, whereas the "at" version simply means that he writes at home rather than in any other location. "He used to write his fiction at the office, but now he writes at home".
I'm not too comfortable with "writes fiction"; it implies that he writes fiction at home, and/but non-fiction somewhere else. I'd feel better with"he writes at / from home", or "he writes his fiction at/from home", or "he writes his novels at/from home".