The tall man in grey was sitting at his usual place, the corner table facing the main entrance by the window. The mousy lady-librarian was sure the book he was working with was The Mystery of Mr. Bernard Brown (1896) by Phillips-Oppenheim.
She waited till he walked out, and then walked to the shelf marked spy-fiction; the book was in its exact spot. She took the book home.
Tracing the erased pencil marks, she prepared a note and posted it the following morning.
Its deciphering in a backroom in Washington precipitated a flurry of calls between White house and Pentagon.
Thanks for the prompt Rochelle Wisoff-Fields. SHALOM.