The Light In The Darkness Book 1: The Secret In The Plot - The Light In The Darkness, #1

In the midst of Zack and Kelly's complicated lives, something evil and dark is brewing in the spiritual realm that co-exists alongside the human one they reside in. A spirit named Ryan is recruiting spiritual energy to completely destroy the barriers that separate the human realm, spiritual realm, and the abyss; a place where the demented, banished spirits have long awaited for their 'Savior' to release them into the human realm once again.
For Ryan to gain the amount of power it would take to do so, he has to take over Zack's body completely. When Kelly finds that Zack has been attacked in his own home by Ryan, she decides to go along with a plan that was concocted by a spirit that has been haunting her for weeks. They decide to banish Ryan from the realm of the living permanently before he gains the power he needs to tear down the barriers, and before the human realm crumbles under the pressure of the excess energy that would be released from the abyss.