Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Gallows Humor / Urgent / Hopeful
It's probably going too far to call Code Name Verity a funny book, but what's unique about it is that the author has managed to create a character, Julie, who maintains a sense of humor in the middle of a terrible situation. We can't think of many other Nazi-themed books in which the protagonist always seems to be laughing at her captors, and even at herself, despite her impending doom.
She describes an exchange between Engel and Thibaut as "a truly hilarious moment" (1.22.XI.43.29), which is hard for us to wrap our brains around, but so it goes—we've also never had to navigate the Gestapo. Julie's sense of humor is a huge part of her strength and what keeps her alive long enough to finish her story.
In both parts of the book, there's a sense of urgency, a sense that we are building toward a climax (Julie's death), but also a sense that something needs to be accomplished. We don't find out what that something is until we discover that Julie's true mission was not to be a wireless operator, but to blow up the Ormaie Gestapo building. Throughout both parts, the narrators are trying to accomplish this goal.
There's also a hopeful tone in the book. While Julie knows she will probably die very soon, she's still hopeful that she can complete her mission before she goes, and she's sure that the Nazis will eventually be defeated. Even after Julie's death, Maddie finds hope in Julie's writing and in her relationships with Julie's mother and brother. Maddie writes of Julie's narrative:
And this, even more wonderful and mysterious, is also true: when I read it, when I read what Julie's written, she is instantly alive again, whole and undamaged. With her words in my mind while I'm reading, she is as real as I am. [...] She's right here. Afraid and exhausted, alone, but fighting. Flying in silver moonlight in a plane that can't be landed, stuck in the climb—alive, alive, ALIVE. (2.20.6)
Julie might be dead, but her legacy lives on—and though her death is a major bummer, there's something hopeful about that.