The Lady and the Thief by Kate Moore

In 1835 London, companion Vivian Bradish daringly pursues her dream of being a writer by researching and notating her ailing employer’s new book–A Guide to London for Fearless Women. Posing as a mark, Viv boldly enters a street known for pickpockets. When she’s attacked, she accidentally shoots the handsome gentleman who aids her. She brings the wounded stranger to her employer’s home for medical aid, but they are caught in a compromising situation, threatening Viv’s position and dreams.
Her would-be rescuer, Lark, was once a Lost Boy, part of a gang of street urchins and pickpockets rescued by the Duke of Wenlocke. Reformed, he’s now Edward Larkin, posing as a gentleman and searching for clues to his past. Meeting Viv is unexpected, and to save her position in the household, he proposes a fake betrothal.
To “court her,” Lark promises to guide her through the streets of the London she’s never seen so she can finish the book, never dreaming his impulse to help Viv will lead him to his lost family.