The library can be a place of intrigue and mystery - that's what librarian Georgia Grainger found out one day, when a little old lady approached her with a question.
"Why does page 7 in all the books I take out have the 7 underlined in pen? It seems odd," she said.
Odd indeed! And when Grainger checked other books, she discovered that many, though not all, also bear the secret code.
So why are some books marked with the secret code and not others? Grainger reveals the deviously clever reason here.