The Rogue and the Runaway (Fallen Angels #4)

The unusual heroine, Maxie, is half Native American and doesn't fit in too well with her British relatives at their country home and comes to suspect that her uncle is hiding something about her father's death. She decides to go to her other aunt in London and sets out on foot from the country estate.

Robin, a retired spy, is restless on his brother's estate and not over distressing events in France. He encounters Maxie on the road and rather than whistling up a coach decides to accompany her in the persona of a footloose ne'er-do-well.

Of course it is the duty of a good English family to protect their young women, even the strange half-Indian ones, and when Maxie's aunt shows up for an unexpected visit she immediately joins in the hunt and tracks Maxie far enough to find out that she is travelling with a known rogue and harangue Robin's brother about it. Add a determined Bow Street Runner and realize that they're hunting a slippery Indian girl and an equally slippery British spy.