Fool Errant — Patricia Wentworth

Political intrigue and industrial espionage are brewing in Britain's Foreign Office in this thriller from the author of the Miss Silver Mysteries
 
On a dark, foggy night, Hugo Ross encounters a beautiful woman. She claims to be running away and begs Hugo not to tell anyone that he's seen her. Before boarding her train, she warns him not to take the job he's applying for: secretary to eccentric inventor Ambrose Minstrel. The train pulls away, and the stunning stranger is gone.
 
Desperate for employment, Hugo ignores her warning and takes the job. He's barely moved into Meade House when a message from Loveday Leigh is hand-delivered: He must leave immediately and burn the letter. When they finally meet again at Waterloo Station, Loveday is not the mysterious woman Hugo remembers. Odd happenings continue, and he enlists the help of the esteemed Benbow Smith, an enigmatic figure connected to London's Foreign Office. Soon Hugo is caught up in an undercover plot involving governmental intrigue, industrial espionage, and stolen military secrets. With his own life on the line, how much is he willing to risk for his country?
 
Fool Errant is the 1st book in the Benbow Smith Mysteries, but you may enjoy reading the series in any order.