See Delphi and die : a Marcus Didius Falco novel / Lindsey Davis.

Enhanced descriptions from Syndetics:

It's 76 A.D. during the reign of Vespasian, and Marcus Didius Falco has achieved much in his life. He's joined the equestrain rank, allowing him to marry Helena Justina, the Senator's daughter he's been keeping time with the past few years. But that doesn't mean all is quiet for Falco, Helena, and their two young daughters.  By trade he is an informer, a man who looks into sticky situations, and he's been hired to pry his errant brother-in-law away from a murder investigation. Which means Falco must himself take it on -- requiring that Falco and Helena travel to Olympia in Greece under the guise of being tourists interested in the classic sites to investigate the suspicious goings on and the shady dealings of a fly-by-night travel agency. With two woman already missing from the packaged tour, things only get stickier when two more - including Falco's brother-in-law - disappear in what is Falco's most complex and dangerous case yet.