E. Phillips Oppenheim 
The Zeppelin’s Passenger [EPUB ebook] 

A World War I tale of espionage in a fictional “backwater” area in England with no apparent military value called Dreymarsh. The story begins with Dreymarsh residents discovering an observation car from a German zeppelin along with a Homburg hat near Dreymarsh. This discovery triggers a range of events and introduces a myriad of characters which all make for a rather delightful mystery. Oppenheim was famous for his hundreds of spy and espionage novels. He is considered one of the originators of the thriller genre, his novels also range from spy thrillers to romance, but all have an undertone of intrigue.
The Zeppelin Passenger is the most audacious German spy story with a strong hint of romance that Mr. Oppenheim has heretofore written.