Lady Molly of Scotland Yard by Baroness Emma Orczy (1865 - 1947)

Scarlet Pimpernel, but who also invented two immortal turn-of-the-century detectives in The Old Man in the Corner and Lady Molly of Scotland Yard. First published in 1910, Orczy’s female detective was the precursor of the lay sleuth who relies on brains rather than brawn. The book soon became very popular, with three editions appearing in the first year. As well as being one of the first novels to feature a female detective as the main character, Orczy’s outstandingly successful police officer preceded her real life female counterparts by a decade.(Introduction from Wikipedia)