An ISBN was a book identification code.
The ISBN for Emily Dickinson: The Complete Poems was the passcode Suzie Costello created to unlock the Torchwood Hub after being put into lockdown from the recitation of "The Chariot". (TV: They Keep Killing Suzie)
Panda once noticed that some of the books being sold in the cathedral gift shop didn't even have ISBN codes. (PROSE: Iris Wildthyme and the Unholy Ghost)