I went back to the library today and as you can see, they changed the sign on the display and also sent me the e-mail below. I didn't think they would change it, but happy they did.
"Ms. Veale, When I spoke with the staff person responsible for the display, she was horrified that she caused you pain. She had seen the phrasing used as a title of a book and thought that it would be intriguing to our mystery readers. She immediately changed the sign to read - Have yourself a merry little mystery. We are sorry that the wording was disturbing to you and are so sorry for your loss. We want the library to be a place of comfort and never one of distress."
I am so happy they changed the sign and proud of you for bringing some awareness to people of how a sign like that affects those of us who have had our loved ones murdered. I had a feeling they would change it.