Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Light-hearted
The book might be about the end of the world as we know it, but the tone is light-hearted. Heck, it even pokes fun of people at times, like when Gratuity describes the panic about going to Florida:
It was Moving Day, and everybody was crazy. You remember. It was chaos; people running around with armfuls of heirloom china and photo albums, carrying food and water, carrying their dogs and kids because they forgot that their dogs and kids could carry themselves. Crazy. (1.7)
You might have noticed how easy-going and even jovial Tip's tone is. She's not freaking out or running around screaming, even though aliens have overtaken the planet and forced humans to gather in one place. In fact, if we didn't know any better, we'd think she was just describing some nut at the mall selling churros. And since she's the narrator, so it goes throughout the book. Yet the whole book is about the scariest time of her life. If you ask us, Gratuity uses a really laid-back tone because the book is so dark and twisty—maybe it's her way of coping.