Ghosts of Belle Isle by Steven K Smith.

Brothers Sam and Derek can’t wait to explore historic Belle Isle during the 150th anniversary of the Fall of Richmond and the end of the Civil War. But that’s before they learn how the wooded park along the James River once held Union soldiers in a terrible prisoner-of-war camp. Legend says that the haunting lights over the rapids at night are the ghosts of long-dead soldiers still fighting the Civil War.

The discovery of a haunting mural in the island’s crumbling ruins leads the boys and their friend Caitlin into danger. To find the truth, they must brave menacing bullies and an encounter with a Confederate biker gang led by the notorious Mad Dog DeWitt. But some ghosts may be more alive than they ever expected.