The Great Game - The Londum Series, #7

Book Seven of The Londum Series, set in an Alternate Victorian England, called Albion. Sometimes you can do bad things for good reasons. Ask Jim Darby, he knows better than most. After helping his lady friend, the Honourable Fiona Hetheringham, with a little insurance scam involving the staged theft of the famed Hetheringham Necklace, he's clandestinely approached by the British Secret Service, in the form of Thornton Wells, to do a little 'search and recovery' operation for them.
A secret treaty between Rooskia and the Ostro-Ungarian Empire needs to be recovered from the Rooskian Embassy in Budapescht, so Jim obliges and hands it over to his Ungarian contact. Job done! But suddenly the treaty has disappeared, his contact has gone missing and everyone's pointing the finger at Jim. Jim has no option but to go back to Budapescht and sort it all out for himself. And just to add to the fun, Jim's old adversary from Caledonia Yard, Inspector Herbert Corner, is on his trail for the Hetheringham Necklace robbery.(There are times, Jim feels, when a change of career to Merchant Banking can seem awfully attractive.)