The Mysteries of Hawthorn Hall by Mindy Burbidge Strunk

She doesn’t trust him. But he is the only one who can save her.

Lily-Anne Cole never fit in with the other maids. But when she inherits an estate, she doesn’t fit in with the ton, either. Matters only become more complicated when a wild-looking man breaks into her house. Luckily, a handsome groom comes to her rescue. Or at least she believes he is there to rescue her.
Timothy Hastings has made himself into a successful, almost-gentleman by cleaning up the messes of the English ton. When he is hired to find a treasure of stolen diamonds, everything goes wrong from the beginning, and he finds himself playing the role of a groom. He only needs to deceive the mistress of the house long enough to find the treasure and receive his payment for the job. When Tim’s deceit is uncovered, he wonders if this will be the first time he doesn’t complete an assignment.
Compelled to spend more time together searching for the elusive diamonds, neither Tim nor Lily can deny the draw they feel for each other, even as they realize a connection can never be. But when others come searching for the treasure and threaten Lily’s safety, they must decide if they are better together or apart