Variations was a book written by David Banderson. Every time it was read, the story would change.

The Tenth Doctor and Donna broke into Banderson's house when there was no answer at the door, and they discovered he was being influenced to write stories by a creature in the computer screen that was kept alive by stories. Donna was nearly caught in the creature's trap too, but the Doctor intervened, freeing Banderson from the creature. (PROSEOnce Upon a Time)