Theodosia and the Staff of Osiris

Theodosia and the Staff of Osiris by R.L. La Fevers
When mummies go missing all over London, eleven-year-old Theodosia puts aside her fight against the Serpents of Chaos to save her father, who is suspected in the thefts, all the while avoiding a string of new governesses. Books in this series: Book 1. Theodosia and the Serpents of Chaos -- Book 2. Theodosia and the Staff of Osiris.

The Case of the Missing Marquess

The Case of the Missing Marquess by Nancy Springer
A new series featuring Sherlock Holmes' much-younger sister Enola begins. When her mother disappears, Enola is determined to find her and sets out to the heart of London. There Enola becomes involved in the kidnapping of a young marquess.