Secret service
Free to read here, from Project Gutenberg.
The Riddle of the Sands
Secret armies
Secret Service Under Pitt
On Secret Service
Spies and Secret Service
The German Spy System from Within
British Secret Service During the Great War
Twenty-five years in the Secret Service: The recollections of a spy
The Red Room
The World's Greatest Military Spies and Secret Service Agents
John Dene of Toronto: A Comedy of Whitehall
The Radio Boys on Secret Service Duty
A Rogue by Compulsion: An Affair of the Secret Service
The Sky Pilot's Great Chase; Or, Jack Ralston's Dead Stick Landing
The Sky Detectives; Or, How Jack Ralston Got His Man
My adventures as a German secret agent
Wings Over the Rockies; Or, Jack Ralston's New Cloud Chaser
Eagles of the Sky; Or, With Jack Ralston Along the Air Lanes
Trackers of the Fog Pack; Or, Jack Ralston Flying Blind
Flying the Coast Skyways; Or, Jack Ralston's Swift Patrol
The Young Wireless Operator—With the U. S. Secret Service
Direct methods
Affair in Araby
The Return of Clubfoot
Walker of the Secret Service
Britain's Deadly Peril: Are We Told the Truth?
La police secrète prussienne
