Legend of the Highland Dragon by Isabel Cooper

The MacAlasdairs are not like other men.

Descendants of an ancient alliance, they live for centuries, shifting between human and dragon forms. Some wander the earth; some keep to their lands in Scotland. And Stephen MacAlasdair, the newest lord of the family, must go to London to settle his father’s business affairs. He brings an object of great power and greater darkness. He finds an enemy from his past, whose wrath is still living and deadly. And he meets an ally he’d never have expected.

1894 London, doesn't provide an easy life for women of the lower class, but Mina Seymour has managed to work herself up to a position as the secretary of a famous scholar. When a tall, dark Scottish stranger demands to see her employer, Mina is irritated; when MacAlasdair's departure leaves the professor worried, she’s suspicious. Determined to figure out the situation, she investigates further - and finds a world and a man she could never have imagined.