Crude Currency by Gary Stull

When cruise ship investigator John Lock agrees to help an old friend find his missing brother--an EU negotiator on assignment to Turkey-- he has no idea what he’s up against.

The nature of August Wolfe's assignment is as vague as his circumstances. It is rumored the missing negotiator was sent to the Middle East to secure a secret oil deal. Evidence suggests he’s been dealing with the mob as well as corrupt government officials. But is Wolfe on the run, or is he the victim of foul play? Whatever the truth, Lock is determined to find him and bring him home.

Before he can act, Rachel McKean -- a tenacious EU investigator and Lock's former lover -- reenters his life. She was dispatched by the European Directorate to track down August Wolfe. Often at odds, the two detectives join forces to follow a trail that leads from Istanbul's Grand Bazaar to the coastal mountains of southern Russia, from Sochi to Trabzon, from Athens to Paris. Time and again their path is barred by crooked politicians, human traffickers, drug smugglers, and the Russian mob. But what have these groups to do with the disappearance of August Wolfe?

Pitted against white slavers and black-marketeers, the erstwhile partners struggle to save a man who may not be worth saving.

In his most ambitious case yet, John Lock tests the bounds of loyalty and friendship. How far will he go to keep a promise and satisfy his own sense of justice?