Is John Shooter from the book "Secret Window, Secret Garden" by Stephen King real or imaginary?

In the book "Secret Window, Secret Garden", after Mort dies, Amy found John Shooter's trademark hat; she took it out to the trash, and planted it right-side up on a trash bag. When she returned, she found a note from John Shooter inside the overturned hat, revealing that he has travelled back to Mississippi. Since Mort has been SHOT DEAD, it clearly cannot be Mort planting the note inside the hat, cause HE'S DEAD, does this mean that John Shooter actually was a real person?