Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Informal; Conversational
From the first sentence of the book, "There was a boy in her room" (1.1.), Fangirl talks to you like you're a close friend having coffee—which, if Levi has anything to do with it, will be a Starbucks gingerbread latte. The book is frank and honest about who characters are and what they're going through—no formalities or pretense here—and the whole thing reads like conversation between friends. Let's look at an example, though, to really sort this whole tone business out.
On the one hand, Reagan didn't seem interested in staying up all night, braiding each other's hair, and becoming best friends forever. That was a relief.
On the other hand, Reagan didn't seem interested in Cath at all. (3.7-8)
We picked this passage for a couple of reasons. (1) It's not a conversation, so all the better for establishing the conversational tone of the book (fast fact: Conversations are pretty often conversational… weird, we know); and (2) it's an excellent demonstration of how informal the tone of the story is.
When you read the excerpt above, doesn't it sound like something your best friend might confess to you about her new roommate at college? The language and pacing is totally similar to how we speak to our friends—try saying it all out loud, and we're sure you'll see what we mean—and what's communicated isn't just facts, but also feelings. We're not opposed to a conversation that's just about the facts, but when we call our besties, there are usually feelings up in the mix as we talk about our lives.
And that brings us to the informality of the tone. This narrative isn't guarded, it isn't keeping secrets or concerned with politeness, and this informal approach to the story comes through in how it's told. In fact, if you think about it, it would be pretty weird for the tone to be conversational and formal. Like, how would that even work? We'd certainly get way fewer insights into the insecurities and struggles of the characters, and part of the fun of reading a coming-of-age story is going into the trenches with the characters. Best to keep it informal, then.