Beyond The Door is a bridge piece between Revenant and Geist. 
Zoë is trapped in the Abysmal Plane and Dags is little more than a doll with the pages of the Grimoire inside of him scrambled, many of them missing. Zoë and Dags' friends believe the Grimoire is the key to helping Zoë, but first, they have to find the missing pages and put them back, in order. 
But Zoë's mother, Nona, has decided that Dags' body is better kept at the Society house under heavy guard, convinced by her ex-husband, Zoë's father, and Ethereal Virtue, that Dags' half-life would be better used as the key to taking control of the Abysmal Plane.