Devil's Fork's underlying theme is about transformation. The lead, Roger Jolly, is a good man, a principled man, yet one who has bounced from woman to woman for so long, he's lost touch with what a relationship can be like. During the course of of the book we watch Jolly develop a meaningful relationship with Jeannie, only to have it disrupted by a sniper who reigns chaos on his rafting excursion.
Still, we watch Jolly transform as he puts the life of a woman he barely knows, ahead of his own, and he pays a hefty price to do so.