Troglodytes - P.I. Frank Johnson Mystery Series, #4

When American diplomat Sylvester Mercedes flies into Ankara, Turkey on what is supposed to be a normal business trip, he mysteriously vanishes from his hotel room. With her husband now missing for more than two weeks, prominent socialite Lois Mercedes hires P. I. Frank Johnson to track him down. Frank had spent some of his Army MP days in Ankara 14 years ago, and while he's not at all happy about going back, being nearly broke can be a powerful motivator.
But after getting mugged and finding a murder at the same hotel, Frank manages to track the clues to the kidnapping down into the troglodytes' subterranean cities in Cappadocia. And that's when things really get interesting. Frank hates dark, low places, but he can't afford not to go down the maze of tunnels if he's to close out the case, collect his paycheck, and return home alive.