Edward Phillips Oppenheim 
The Kingdom of the Blind [EPUB ebook] 

Espionage and Counter-Espionage
World War One.
Surgeon-Major Hugh Thomson is engaged to the beautiful Geraldine Conyers. Ostensibly responsible for military hospitals on the front in France, in reality he is in charge of military counter-intelligence. He is on the trail of a German master spy who seems to be able to travel across the lines and back and forth between Germany and England.