- (3/-1) Trot and Cap\'n Bill from The Sea Fairies appear in this author\'s novel \"The Scarecrow of Oz.\"
- (6/-2) This author\'s best-known character visits his Pellucidar series of novels in \"Tarzan at the Earth\'s Core.\"
- (9/-3) Based on science-fiction author Theodore Sturgeon, this Vonnegut character appears in \"Breakfast of Champions\" and \"Slaughterhouse Five.\"
- (12/-4) A character in this author\'s Foundation series makes reference to Nemesis, another novel written by the same writer.
- (15/-5) In his novel The \"Number of the Beast,\" characters visit several of his fictional universes including that of \"Stranger in a Strange Land.\"
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