What does "May your death only ever be written" mean?
It was written by a mystery novel writer. She signed this on her book to show appreciation to the detective who solved a real crime and proved that the writer wasn't the murderer.
This statement would be similar in meaning to "May you live forever." Since the writer is an author of murder mysteries, it is a genre-appropriate well-wish to the recipient that she wishes that the only death they would ever experience is the one that is (fictionally) written about them.