Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Gloomy
Before we even know what's up with Finnigan and Gabriel, we know something bad is about to happen. How? The tone is awfully gloomy. Check out how Gabriel describes himself:
In the center of my palms spread two dark lakes. They are not wounds—flesh ices them over. But the blackness is blood, gathered swampishly. When I press a finger to the fluid, it radiates. When I clamp and unclamp my fists, it dissipates. (13.2)
We know the guy's dying, but this is somber even for someone on their death bed. It's true that Gabriel no longer wants to stick around, but his tone makes sure we know it. The gloom-and-doom approach also helps keep the suspense alive. Even when nothing's happening with our ongoing mystery of who Finnigan is and how he connects to Gabriel, we feel destined to be frightened with the melancholy tone.
What fascinates us most about the tone is how both Gabriel and Finnigan share it. Did you notice how the book is from both of their perspectives, but it sounds exactly the same? When we switch back and forth, we hardly notice. Don't believe us? Check out this passage from Finnigan:
I put my fiery hands in my pockets, walk away from the crackling light. Cross the lawn to where the night's gathered black as mortal sin. Surrender, the dark's dog, follows me. We sit down with our spines to the wall. (14.3)
Here we see Finnigan using the same tone and style as Gabriel. This is a clue that they are the same dude long before we know it for sure. Clever, that Hartnett.