Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Sarcastic and Curious
Since Kiriel is the one telling us the story, the book's tone is his tone. We don't think we're dropping any major bombs when we say that he's a sarcastic guy—just pick a page and let the cynicism flow. For example, he says, "This time, I didn't just deserve punishment. I owned it. I reveled in it. The consequences were mine. Take that, Creator!" (5.24). Looks like someone's feeling a little saucy today.
On the other hand, there's a part of Kiriel that's very curious. He wants to explore and figure stuff out—he wouldn't have come above ground otherwise. For instance, he tells us: "I have never been anything but spirit—anywhere and everywhere I wanted to be, just never in a physical sense. This was the first time I was ever in exactly one place. Before, I could have known what anybody on earth was doing, if I'd felt like it" (1.15). A lot of the tone of the book is showing him do exactly that, testing stuff out and thinking about how it makes him feel.
It's almost like Kiriel's on an adventure. You know, if the adventure involves stealing a body and using magical powers. Hmm… come to think of it, that does sound adventurous.