What was surprising to me (you mention Maugham and Secret Service) was how many writers were involved in British Intel work, including Le Carre, Fleming and Forsyth. One of my favorite writers, in this regard, was Graham Greene. I loved the way he produced such insightful stories in the various settings (Third Man, Quiet American, etc) and only later did I connect the dots that he was undercover in Nam, and post WW2 Vienna, Haiti and elsewhere.