Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Wistful
For all the darkness of some of its moments, for all the sorrow it explores, this book is wistful. It's about longing: for home, for healing, for love.
Take this scene from the novel, where Ada is watching a sunset and thinking:
Meanwhile, the book still in her lap, Ada remained on the porch looking out across the yard and down to the barn. On past the fields to the wooded slopes. Up to the darkening sky. The colors that had reminded her of Charleston were now muted. Everything declining toward stillness. Her thoughts, though, seemed intent on folding back on themselves, for she recalled that she and Monroe had sat thusly on a night just after moving to the cove. These now-familiar elements of landscape had seemed strange to them both. (6.46-47)
That's Cold Mountain for you. Yearning and wistfulness are central to it. The characters differ on whether that yearning points to a world beyond our own, or whether it simply wells up while we're observing this world. But either way, longing is a central theme: Inman's longing for Ada and hers for him. The longing of Monroe for the woman who became Ada's mother.
And that yearning is present in not only the story the book is telling, but in the way it is told: Frazier uses imagery and metaphor to make that longing real to the reader. The spare but striking description of the sunset in the above passage is just one example.