The Third People — Lee Burton

What if they've always been there, hiding in the dark, waiting? A new group threatens the territory of young Jori's tribe, but it is not the band of second peoples they track to the river. It is the mysterious third people, Jori learns, who dwell in the land of the night. And now, Jori's father tells him, they must cut the stone points from their spears, sharpen their tips, and prepare to fight for their survival.