The Spy’s Convenient Bride by Erica Taylor

No longer an agent of the Crown, Luke Macalister doesn’t know who he is outside the clandestine world he spent the better part of a decade in. When he is awarded an earldom for something he didn’t do, he sees only a future of boredom and responsibility as the new Earl of Kenswick, a future he is desperate to escape.
The daughter of the late Baron Kenswick, Miss Vivian Burke despises everything about the new earl she’s never met. That he turns out to be handsome and charming, and incapable of taking anything seriously, does not elevate him in her opinion. Until he offers her a deal—he will hand over the deed to her childhood home in exchange for a six-month marriage of convenience. To save herself and her mother, she has no choice but to say yes.
Their plans of a boring six months in London are soon derailed as Luke’s life as a spy catches up to him and he’s left with no choice but to trust his new wife.
Armed with only a diary and a signet ring, Luke and Vivian race against treasonous forces to save a life, to save England, and if they’re lucky enough, they just might save each other and find love along the way.