Case of the missing marquess: an Enola Holmes mystery. Nancy Springer. 2018.

When Enola Holmes, sister to the detective Sherlock Holmes, discovers her mother has disappeared, she quickly sets off to find her in London - but nothing can prepare her for what awaits. For when she arrives, she finds herself involved in the kidnapping of a young marquess, and must flee murderous villains and elude her shrewd older brothers - all while trying to piece together clues to her mother’s disappearance. Amid all the chaos, can Enola find the marquess and her mother?