What does the third letter from Tommy say in Chasing Vermeer?

In the book "Chasing Vermeer" by Blue Balliett, the third letter from Tommy is a coded message that reads: "The lady in the window is not what she seems." This message is significant in the story as it refers to a mysterious painting that the characters are trying to unravel the secrets of. The letter hints at hidden meanings and deception surrounding the artwork, adding to the intrigue and suspense of the plot.