J. S. Fletcher Murder of the Secret Agent: being Entry Number Eight in the Case-Book of Ronald Camberwell (London: George G. Harrap & Co. 1937).
J. S. Fletcher turned out this sort of stuff by the truckload from the mid-1910s until his death in 1935. His most famous work is The Middle Temple Murder (1918; free on Project Gutenberg). I've not read it yet, but after reading Murder of the Secret Agent (1934) I shall go and find myself a 'real' copy.
Murder of the Secret Agent is a typical specimen of the period (it is set in 1930) with the manly hero, his offsider with police connections, a helpless heroine ("handsome, healthy, with undeniable traces of the modern public-school product about her"), a dastardly Russian princess, an American millionaire, forged papers, devious twins, a dodgy parson, a fortune in stolen jewels and a murder on a lonely moor: all wonderfully cinematic. And this is proper old-fashioned detection: the moment the detective pulls out his Bradshaw's Railway Guide you know you're in safe hands.
It has aged quite badly, but is, I would suggest, all the more delightful for its evocation of a gentler, lost world. What more could you want to fritter away a few hours? My only puzzle is the title: no spies herein; but it was certainly catchy enough to make me remove it from the shelf.
If you liked this... hmmm. Maybe something like Roy Vickers' The Department of Dead Ends (1947: short stories, many from the 1930s). Something with more tedious train trips would be even better.