Ms. Flanagan writes fan fiction—amateur works based on the characters and settings from novels, movies, television shows, plays, videogames or pop songs. Such stories, which take place in fictional worlds created by professional writers, are flourishing online and attracting millions of readers.

Ms. Flanagan started writing her own takes on "Twilight" three years ago, after devouring Ms. Meyer's vampire books. She has since written 15 stories, including some that are as long as novels. In the process, she has gained groupies of her own. Some 1,500 readers subscribe to her account on fanfiction.net.

"A lot of people don't understand why I would devote time to this," says Ms. Flanagan, who writes at night after her young son goes to bed. "It's just fun."