You may be looking for another short story of the same name.
Once Upon a Time was a story printed in Doctor Who The Official Annual 2009.
Summary
The Tenth Doctor and Donna discover a book that changes every time someone reads it. Intrigued, they track down the author and find him in the process of writing the very story that led them to him. But the author cannot stop writing.
Characters
Worldbuilding
to be added
Notes
to be added
Continuity
to be added