Author is Emma Lathen, and this is the ninth book in the John Putnam Thatcher series.
When the Greek colonels' revolution breaks, the Sloan Guaranty Trust, with thirty million dollars invested in a hydroelectric project, finds that its representative has disappeared. A trust officer sent to replace him also drops out of sight. Enter the company's senior vice-president, John Putnam Thatcher, to set matters right. With the help of two lady archaeologist doubling as partisans, the unflappable Thatcher pulls off a coup of his own in the wildly funny and exciting climax.