Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Sardonic, Self-Deprecating, and (a little) Despondent
Although the story is told through third person narration, the narrator is very close to Nick throughout the book, which means we see things from Nick's point of view and get a lot of his inner thoughts. And those inner thoughts? They're pretty dark. And funny. And a little sad.
Because Nick isn't overly fond of people in general, and because he doesn't always know how to interact with them, he tends to relate to others using sarcasm and exceptionally dry humor. Like when he tells Alan, "Oh get in the kitchen and bake me a pie, woman" (1.6), or when he cautions Jamie, who's fidgeting with an oven glove, "Don't hurt yourself with that" (7.52). And while Nick may seem arrogant or self-centered at times—as when he refers to himself as "stupidly good looking" (7.47) or comments glibly on Mae's breasts (7.79)—he's also quite aware of his own flaws (the anger, the apathy, inability to communicate), and more than ready to point them out.
The result is a great mix of humor and despair that keeps us entertained as we chuckle and cringe our way through the book.