Can anyone help me identify a book I looked at briefly then lost my note of title and author? It's a mystery. The setting is Britain, time uncertain. The story involves a pair of investigators. She is high society and welcomed in homes all over London. He, on the other hand is a commoner, perhaps even a policeman, and must go 'round to the servant's entrance. It may be one of a series of mysteries by the author. Thanks for your help!