The Highland Earl by Amy Jarecki

Mr. & Mrs. Smith meets Outlander in this action-packed Scottish romance where a marriage of convenience leads to secrets that could be deadly.
With two young sons to care for and a large estate to run, John Erskine, the widowed Earl of Mar, needs to remarry as quickly as possible. And beautiful Lady Evelyn Pierrepont would be the perfect match. But there’s more behind the English lass’s calm demeanor than she’s letting on, including a smoldering allure not even John can ignore.
Evelyn has no desire to wed the rugged Scottish earl, but at least she’ll be able to continue her work as a spy-as long as her husband never finds out. Yet the more time Evelyn spends with John and his boys, the fonder she grows of their little family, and the last thing she wants to do is put them in danger. As alliances shift and enemies draw closer, soon everything they hold dear is at risk: their lands, their love, and their very lives.