What to read after Winnetou's Heirs?

The aged Karl May and his wife receive invitations from Indian friends and foes in America, to participate in the decision making on a monument to Winnetou. May steps into Old Shatterhand's shoes one last time, and when they arrive at the Indians' gravesites, Old Shatterhand excavates Winnetou's long-lost legacy. In the meantime two opposing groups of Indians are gathering at the proposed location of the monument. Covertly, a large group of enemy warriors approaches through an extensive cave complex to attack, but natural events intervene. A geological catastrophe provides for an unexpected turn of events.