Surprising Lord Jack by Sally MacKenzie

Frances Hadley's aunt is trying to sell her into marriage. But... the guy is a scumbag. So she does the only rational thing - dress like a boy and run away. Only, she doesn't get very far. Her horse comes up lame and with the rain... she ends up in a room at an inn. It's a very full inn and the room is generally reserved for the Valentine family. But there's a big party at their house so they probably won't need it.
Except... that Jack Valentine, the youngest son, is trying to escape his mother's machinations. So, he ends up sleeping with someone he thinks is a young boy. No problem. It happens during this time period.
Except... someone recognizes that Frances is not a Francis and, when Jack realizes that, he realizes that she is compromised. Apparently, not so much for sleeping with him but for sure being dressed in boy's clothes. But, she's not really traveling in their circles so Frances doesn't see this as an issue.
Except... there's a slasher in town and he's killing "unchaste" women, a category into which Frances seemingly now falls.
A fun romp and a cute love story. I'm really enjoying this series and can't wait for the next book, Loving Lord Ash.