There's no classification for fan fiction? Fanfiction has been around for generations! Surely, there are scientists with a passion for fan fiction and its history, how have they not carved out of space for it? Is there not even a debate about this?
Specific books about fan fiction have been categorized under 809.3 (literary criticism of fiction), 808.3 (rhetoric of fiction), 813.6 (american fiction since 2000), and 346.048 (intelectual property law). I can't tell you why, cause I don't know.