Hmmm... off the top of my head, it would be a book of horror and history, and a film documentary afterwards.
The story: A accountant name Joey in his late 50's goes on a camping trip with his son-in-law and grandson. While fishing Joey gets bitten by an odd looking snake. He's taken to a hospital, where he dies for a couple of minutes before doctors bring him back to life. All he remembers of the afterlife is many ghosts of musicians vying to touch him. He dismisses the vision and some time after his recovery he is given tickets, with back stage passes, to a small concert venue that is having the Pretenders playing. Back stage he is left alone and finds a bass the group keeps that belonged to deceased member Pete Farndon. As soon a Joey picks up the bass he is possessed by the spirit of the bassist at 30 years of age. Possessed Joey confronts the Pretenders and is able to answer questions no one else but Frandon would know. He plays a little and convinces the Pretenders to let him on stage with them. When Joey puts down the instrument the spirit leaves his body. The story permeates the rock world and soon he is getting requests from rock bands looking to see if the stories are true. To make a long story short Joey gets to play concerts possessed by John Bonham of Led Zeppelin, Kurt Cobain of Nirvana, Ric Grech of Blind Faith, Phil Lynott of Thin Lizzy, Keith Moon of the Who, Sid Vicious of the Sex Pistols (and a few others King would be interested in). The groups love the experience because they get to relive a bit of their past with the possessed Joey taken over by the spirits of the dead rockers at the age of their death. The horror part involves whenever a spirit leaves, a bit of the demons that plagued the rockers lives remain with Joey, until he finally succumbs to the same fate that caused the deaths of his spirits.
Stephen King would drive Ms Mod crazy by having her contact the rock groups and book all the interviews for King with the remaining members of the bands. King would obtain stories from the rockers and information for the book that have never been told before (the history part), and input how the group would play things out in the story scenario. In real live King would occasionally play songs with the groups at a few of their concerts for the film documentary. The interviews by King with the groups and his playing with them would serve as the basis to the documentary after the book comes out. I figure a 2 - 3 year process. Not that I thought about it much.