Literary Work of Magic

A true artistic work is "revealed" to have been created with some other purpose in mind. Perhaps some god or devil had a hand in it. Perhaps it was a summoning ritual, a la The King in Yellow. Perhaps it was part of a ritual to attain godhood, which is why everyone's so enraptured by it. Either way, someone, or something, other than the author had a hand in it, and we're just finding out for the first time.

Like Beethoven Was an Alien Spy, but applying to the artistry rather than the artist.

This seems to happen to William Shakespeare a lot.

Comic Books

 * The Sandman has William Shakespeare putting on his debut performance of A Midsummer Night's Dream for Oberon, Titania, Robin Goodfellow, and Morpheus. Robin Goodfellow escapes into the world to pester others, and it's implied that Titania is responsible for the death of Shakespeare's son Hamnet. It's explicitly stated that Morpheus commissioned both A Midsummer Night's Dream and The Tempest, in exchange for making Shakespeare a skilled writer.
 * In The League of Extraordinary Gentlemen: The Black Dossier, one of Shakespeare's fictional plays (Faerie's Fortunes Founded) is basically the minutes for the meeting in which the first League was founded.

Anime and Manga

 * The Read or Die OVA features a half-literal Beethoven Was an Alien Spy (the "alien spy" part is figurative) which also applies here: one of Beethoven's symphonies.

Literature

 * In the Tim Powers novel Three Days To Never, it turns out Charlie Chaplin worked symbolic imagery into City Lights as part of a magical ritual to attempt to bring his son back from the dead. An earlier movie he'd worked on but never shown to the public is part of the MacGuffin; Albert Einstein (yes, Einstein, this is Tim Powers we're talking about) had to talk Chaplin out of showing the movie, as the mojo generated by the imagery would likely fry some audience brains.
 * In Songs of Earth and Power by Greg Bear, any sufficiently great work of art is magical.
 * In Dirk Gently's Holistic Detective Agency by Douglas Adams, Samuel Taylor Coleridge was possessed by an alien ghost, who left messages in his poetry.
 * The title character was revealed to be the infamous "A Person from Porlock" that disrupted "Kubla Khan" because, if completed, the poem would've caused the end of the world.
 * Also,
 * Similar to Shakespeare, Dracula is mentioned in almost every Urban Fantasy with vampires, and Bram Stoker usually having an ulterior motive of some kind in writing the book.
 * In The Dresden Files, Dracula was commissioned by White Court vampires for the explicit purpose of teaching humans how to kill Black Court vampires. Because of this, the few surviving Black Court vampires are exceptionally clever and dangerous.
 * The Sookie Stackhouse Mysteries. A short story revealed that Bram Stoker was one of the vampires who wanted to come out early. His sire wanted to be the main character.
 * The Science of Discworld II: The Globe has the wizards of Unseen University visiting the "Roundworld" to fight off the elves as they disrupt A Midsummer's Night's Dream.
 * SOD III: Darwin's Watch uses this trope as well with the wizards trying to make sure that Darwin completes The Origin of Species

Live-Action TV

 * Doctor Who, "The Shakespeare Code": Shakespeare's lost play, Love's Labours Won, was influenced by a trio of aliens to serve as a summoning ritual for their species.
 * Another Doctor Who example is "The Unquiet Dead". At the end, Charles Dickens is inspired to write the episode's monsters into The Mystery of Edwin Drood so the world will know the truth. Of course, he suffers Author Existence Failure before finishing it.
 * In Jekyll, it is revealed that

Tabletop Games

 * Promethean: The Created hints in one sourcebook that the poem "Kubla Khan" was inspired by a qashmallim for purposes unknown. The visitor that interrupted Coleridge and ruined his vision was a Promethean who feared dire results if the poem was finished.
 * New World of Darkness sourcebook Reliquary has Shakespeare's lost play, The Witches, serve as a summoning ritual that opens a portal to... well, it's not a very nice place. The backstory says Shakespeare got the entire audience together after the first performance to promise that it would never be used again.

Video Games

 * Gabriel Knight: The Beast Within has a few scenes revolving around a Richard Wagner opera that was capable of outing werewolves.