A Bride By Morning by Katrina Kendrick

This third book in the Private Arrangements series can be read as a standalone.

A jaded spy meets his match in a spirited wallflower
HE IS A HEARTBREAKER
Gabriel St. Clair, the Earl of Montgomery, has a reputation for being the most charming gentleman in London. But to Miss Lydia Cecil, he is the man who jilted her and shattered her heart. Gabriel would rather Lydia hate him than learn the truth: he’s spent the last ten years as a British spy—and his enemies are closing in.
SHE IS A WALLFLOWER
Lydia has settled into a life as a companion for her elderly aunt. But when she witnesses Gabriel gather intelligence on a foreign enemy, she’s suddenly thrown into his dangerous, bloody game of espionage. To save her life, Gabriel is forced to marry the girl he once abandoned.
As husband and wife match wits, their greatest threat may not be the assassins on their heels, but learning to set aside their pasts or risk losing their love for good . . .