The Albert Gate Mystery by Louis Tracy

A new case for barrister and hobby detective Reginald Brett: The imperial diamonds were sent by the Sultan to London, to be cut in Albert Gate mansion by experts, all the while under the especial protection of the British government. One morning, however, the Turkish officials are found dead in the house, and the diamonds have vanished - despite the strict measures taken to protect them. The first suspicion falls on Jack Talbot, a young secretary at the Foreign Office, in whose charge this mission was, because he also disappeared without a trace on the same evening. Convinced that Talbot is innocent, his friend Lord Fairholme turns to Reginald Brett for help... (Summary by Carolin)