The Case of the Missing Marquess, An Enola Holmes Mystery by Nancy Springer.
As the story opens, Sherlock Homes’s younger sister, Enola, is searching for her missing mother, who disappeared on Enola’s 14th birthday. In the process, she comes across a mystery of a young marquess, which lures her into serious investigating mode. To do the job right, Enola escapes from her older brother’s care into the depths of London, where she pursues her investigation disguised as a widow. Enola is a fully-developed, immensely appealing character, whose pluck and brains are needed for the life she eventually chooses. To achieve what she does despite Victorian restrictions on females is amazing…and inspiring. This is the first of a popular new series by an extremely good writer.