The Innkeeper's Daughter (The Bow Street Runners Trilogy)

A London officer goes undercover to expose a plot against the Crown
 
Dover, England, 1808: Officer Alexander Moore goes undercover as a gambling gentleman to expose a high-stakes plot against the king—and he’s a master of disguise, for Johanna Langley believes him to be quite the rogue. . .until she can no longer fight against his unrelenting charm.
 
All Johanna wants is to keep the family inn afloat, but when the rent and the hearth payment are due at the same time, where will she find the extra funds? If she doesn’t come up with the money, there will be nowhere to go other than the workhouse—where she’ll be separated from her ailing mother and ten-year-old brother.
 
Alex desperately wants to help Johanna, especially when she confides in him, but his mission—finding and bringing to justice a traitor to the crown—must come first, or they could all end up dead.