Dominion — Phaedra Weldon

When Dags and Rhonda disappear without a trace, the prime suspect is the Seraphim, bent on possessing the Grimoire inside of Dags. The Ethereals intend to use the book to seal the Abysmal away forever and rule the Inner Planes unfettered. But what frightens Zoë more is the knowledge the Grimoire's Guardian, Dags McConnell, will die if the book is removed from his soul.  
 
But when no devastating apocalyptic events follow, Zoë and her friends look to their own to find answers that lead them to a new menace from the Ethereal Plane, and a ghost from Zoë's past that reminds her no nightmare is ever, truly over.