Steve Berry 
Balkan Escape ebook 

As a favor to enigmatic billionaire Henrik Thorvaldsen, Cassiopeia Vitt treks into Bulgaria’s Rila mountains in search of a buried stash of exceedingly rare artifacts from a bygone civilization: the ancient tomb of a Thracian king. But when her presence is discovered by a shadowy group of Russians secretly mining the area, she needs a way out. Who to trust becomes the question, and her life depends on choosing the right option.