What does the mayor's wife give to Liesel? How does Liesel respond?

From the Chapter "THE TRILOGY"

Liesel steals another novel from the mayor's home: a green book titled A Song in the Dark. She does so alone and without Rudy. A week later, Rudy brings Liesel to the mayor's home and points out a book that has seemingly been placed intentionally on a closed window. Liesel steals the book, The Complete Duden Dictionary and Thesaurus. As the two leave on bikes, Liesel turns around and sees the mayor's wife Ilsa Hermann standing in the window motionlessly waving to her. Inside the book is a letter from Ilsa to Liesel, in which Ilsa reveals she has known about the book stealing and is amused by it; she invites Liesel to come in through the front door instead of breaking in. Liesel returns to the mayor's house and tries to knock on the door but apparently cannot bring herself to do so.