Tim Lebbon's Firefly novel sends Mal Reynolds chasing a gambling debt turned treasure map, won in a card game and passed to River Tam to decode. What she finds in its coordinates points toward one of the long-lost generation ships that first carried humanity to this system, and Mal can't resist the promise of a payday even as the crew of Serenity is drawn toward a piece of history someone else clearly wants kept buried.
Summary by Loreful, from public sources