Kayleigh Towne is a beautiful and successful singer-songwriter, and Edwin Sharp is her biggest fan.  

When she replies to one of his fan letters with 'XO', Edwin is convinced she loves him, and that her latest hit song 'Your Shadow' was written for him. Nothing Kayleigh or her lawyers can say persuades him otherwise. Then the singer gets an anonymous phone call; it's the first verse of 'Your Shadow' playing. 

Soon after, one of the crew is horribly murdered. Kayleigh's friend Kathryn Dance, a special agent with the California Bureau of Investigation, knows that stalking crimes are not one-off occurrences, and, sure enough, more verses of the song are played as warnings of death to follow. With a little help from forensic criminalist Lincolyn Rhyme, Dance must use her kinesic and investigative skills in an attempt to find the killer before more people die. 

Deaver has written the actual song, 'Your Shadow'.