Andre Gerolymatos 
Espionage and Treason in Classical Greece [EPUB ebook] 
Ancient Spies and Lies

This history of ancient diplomacy demonstrates how the ancient Greeks used guest-friendship as a mechanism of diplomacy. Ancient proxenoi were the equivalent of contemporary consul-generals and they served some of the same purposes. The proxenoi conducted the diplomatic affairs of the state they represented and looked after the interests of the city-state that had adopted them. In times of war the proxenoi maintained spies and supplied intelligence on the movements of fleets and armies.