Derry Greene is a troubleshooter for Tramerica.
That company is a world-wide tour provider and from the sound of it quite large organization, called "one of the giants of the American travel industry". You name a part of the world, chances are better than not Tramerica has a tour that will take you there and make sure you get back.
Which is where Greene comes in. For many years before he went to work for Tramerica, Greene had been a reporter for the Daily Travel News. That gave him a fantastic chance to see a whole lot of the places Tramerica would send people and write about them. As he put it in the very beginning of the first recorded adventure, "I never thought I would wind up working for Tramerica as a sort of combined public affairs specialist and general troubleshooter. But the increasingly turbulent world convinced management to have on hand an experienced man who could handle a variety of sensitive assignments."
He further goes one to say, "As a well-known trade journalist—that is self-flattery but true nonetheless—I had received an inside track on the newly created position. Several other factors were in my favor. With a BA in history from Berkeley, I was considered reasonably well-educated once they were sure I hadn't been a raving student political activist. I spoke some Russian and Spanish—enough to get into trouble was my usual line—suggesting a flair for languages. It didn't hurt, though it wasn't especially cited, that I was still single at thirty-three and had no family. ( My parents were dead, and I had no siblings.) Presumably my intelligence training in the army, utilized on a mercifully brief stint in Vietnam, didn't damage my resume, either."
Considering his experience, why he got and kept the job is pretty obvious. Why his adventures would qualify him for mention in a group of spy series comes because of the variety of troubles that get thrown into his face and pushes him to fight more than just annoyed tourists or peeved hotel concierges. Terrorists and corrupt foreign officials and the odd American government flunky getting in the way will all do their best to make his life not only very difficult but also a whole lot shorter than he would want.