Circus of Shadows by Kimberlee Turley

When seventeen-year-old Gracie Hart gets caught stealing a ride on a circus train, she expects to be arrested. Instead, she is offered a job as the assistant to the circus magician and knife-thrower, Jack. He’s charismatic and genteel but his aim isn’t perfect—hence the position opening.

Once Gracie starts performing with Jack, she begins finding threatening notes hidden in her costume. At first she thinks she’s being haunted by the ghost of the last assistant, but Gracie soon realizes she’s the one trying to warn herself: she’s next.