The idea is inventing, writing and placing the surprising news in a specific context. Cards can be inserted into the books in library or bookstore. Use your creative inventiveness. Below are a few examples.
This can be mysterious message about the content of "I was here tonight" or other scary sentences in a book about paranormal things.
You can fake autograph of a famous person and put it in his biography.
This may be good news? Sure! "I wish you a successful cooking" in cook book.
If you're particularly nasty, write a message like "Ned Stark will die!" and put it in the Game of Thrones.
In the book about dating leave your phone number with your name.
If you have more ideas, share it in comments!