What are Harry Potter's kids up to? Should Jo have married Professor Bhaer? Could Bella Swan have depended less on Edward? For many readers, part of being a fan is the desire to know more than what's spelled out on the page. And when answers aren't readily available, some readers are happy to create them for themselves.

Thus, there's fanart, from the representational (a re-creation of a book's cover image) to the more imaginative (a drawing of Ronan and Blue from the Raven Cycle eating ice cream together, just because). There are fan videos (the full-length parody A Very Potter Musical and its two sequels; a growing number of literary-inspired webseries), there's fan music (an entire genre of wizard rock, or wrock, featuring bands such as Harry and the Potters and their Slytherin-leaning counterparts, Draco and the Malfoys), and there are fan playlists (mixes of unrelated songs chosen to go with everything from A Series of Unfortunate Events to the Lunar Chronicles).

And then there's the seemingly bottomless rabbit hole that is fanfiction. In addition to the hundreds of thousands of stories based on the Harry Potter and Twilight series, there's fanfic for The Giver, The Hobbit, The Baby-Sitters Club, and the Anne of Green Gables books. And the Warriors books (in the tens of thousands). And The Devil's Arithmetic. And The Lorax. Fanfic lets fans take ownership of fictional worlds in any way they want. They can create prequels ("The Fifth Annual Hunger Games"), sequels (Peter Pan returns to the Darling household, but Mr. Darling isn't having it), alternate endings (Rudy survives The Book Thief), or alternate universes (Charlie, Veruca, Mike, Violet, and Augustus attend a school detention supervised by Mr. Wonka). Anyone can write fanfic, and any such writing--anyone's writing--has a chance to change how others think about the canonical work. It's hard to un-see subtext once you've been shown it, and fanfic often infuses the work it's based on with new subtext, romantic and otherwise. After you read a story about Tuck Everlastings Winnie choosing immortality and becoming an alcoholic, you might see her canonical decision not to drink from the spring in a new light.

Usually, that's that. An author creates something, fans create something out of it, other fans (ideally) enjoy it, and everyone waits for the next book. But not always.

SOME AUTHORS ARE willing to engage in conversation about their stories outside the confines of their books. The final Harry Potter book had barely hit the shelves before J. K. Rowling began volunteering more information. Harry and Ron both grew up to be Aurors, she revealed just days after the release of Harry Potter and the Deathly Hallows, and Harry was head of the department. A few months later,...