(en) Penzler, Otto(2020).The Big Book of Espionage. Knopf Doubleday Publishing Group,"Introduction",xi.ISBN 978-1-9848-9806-77.Geraadpleegd op 21 October 2025.
(en) Liulevicius, Vejas Gabriel(October 2011).Espionage and Covert Operations: A Global History. The Great Courses,"1. Introducing the Secret World""Like the old adage about war, real espionage is most often made up of vast stretches of boredom punctuated by sharp moments of fear. [...] It has been said, with some justice that good tradecraft keeps espionage, routine, and boring, which is good, because excitement actually means that something has gone terribly wrong."
