The Passion Project by Laura Trentham

Stargazer, astrologist, tea-leaf reader… Lady Geneva Dorn has heard the disdainful descriptions bandied about for years. In truth, she is a scientist, an astronomer to be exact. And she is on the cusp of an incredible discovery—a new comet.
Assassin, hero, soldier for hire… All true to some extent, but Selwyn McKelvey prefers the term opportunist. He’s brought down villains and despots. Now he’s ready to plant roots at a peaceful cottage by the sea. He just has one last criminal to eliminate. Only the brash, beautiful, extremely intelligent Lady Dorn hardly matches the description of a hardened murderess he was given. Something very inconvenient occurs around her. He develops a conscience… and a weak spot for her sharp mind and gorgeous body.
In order to prove her theory—and finally gain recognition for her work—Geneva has to arrive in Scotland by summer solstice, only one week away. She doesn’t trust Wyn as far as she can throw him (which wouldn’t be far at all), but she needs an escort and he is her only option. It seems more than one faction of the extremely competitive Royal Society is out to steal her research and shut her up for good. But on whose side will Wyn fall?