Where did it start, for you? Maybe it was when J. D. Salinger’s fiction revealed the intertwining of the fates of the siblings of Holden Caulfield, from The Catcher in the Rye, and that of the Glass family from Seymour: An Introduction and others of his fictions (if I recall correctly, Holden’s brother, killed in World War II, was earlier a contestant on the same juvenile game show as Seymour Glass).
Maybe it was when Kurt Vonnegut wandered into Breakfast of Champions, a book that was a compendium of characters both minor and major from his earlier novels, including Vonnegut’s fictional neglected science fiction author Kilgore Trout. Trout was an obvious transposition of the real neglected science fiction author Theodore Sturgeon, whose unlikely name was nevertheless authentic, and who’d been a friendly acquaintance of Vonnegut’s several years before. Bizarrely and wonderfully, Kilgore Trout was also credited with authorship of a mysterious satirical science fiction novel that appeared in 1975, two years after Breakfast of Champions, a book called Venus on the Half-Shell, which turned out to be written by another science fiction writer named Philip José Farmer.