The Case of the Missing Marquess by Nancy Springer

This is an Enola Holmes Mystery--Enola Holmes being the much younger sister of Sherlock and Mycroft Holmes.  It is a delightful look at the late 1800s.

Opening:

The only light struggles from the few gas street lamps that remain unbroken, and from pots of fire suspended above the cobblestones, tended by old men selling boiled sea snails outside the public houses.