These Things Of Darkness by Grace P OBrien
Adam Jedermann is an Outrider; a freelance Bureau control agent with the ability to infiltrate strange worlds and dimensions. Paid to hunt, capture and terminate illegal Visitors who pose a threat to the Continuum, he struggles to cope with feelings of alienation and discover some greater purpose to life.
Lilith, his dead ex-girlfriend, isn’t making reality easier for him. She constantly pops in and out of existence, reigniting their passion and trying to seduce him over to her world.Adam is burning out from Interdimensional Temporal Rejection and is having a hell of a time trying to keep his mind and body together. Haunted by reoccurring dreams and an increased awareness that the world around him is unraveling, he accepts a private assignment convinced it will help him discover what’s causing his troubled sleep. A routine missing persons case gradually becomes an expanding web of mystery and murder, as Adam and Lilith uncover bizarre and threatening forces The Bureau intends to keep hidden at any cost. When the truth begins to shake apart the foundations of Adam’s reality, he’s forced to make a decision that will change his destiny and the world as he knows it forever.