The category 'author mentioned as a minor character' had better exclude frame narrators like Colette, and Lewis in the Space Trilogy, or it will be overwhelmed by Kipling, who probably was in frames within frames, with an informant offering him a new story for the 'sequel' to his in-story-mentioned next collection.

Mark Twain too, I bet. Hm, Huckleberry Finn introduces his book by mentioning Twain and criticizing Tom Sawyer.