His Wicked Charm by Candace Camp

She’s too prim.
Lilah Holcutt dislikes Constantine Moreland from the moment she meets him. He may be handsome, but he’s frivolous, rash, impulsive and, worst of all, a flirt. Now that Con’s twin brother has married Lilah’s best friend, she’s seeing way more of Con than she’d like. And when Con’s sisters are inexplicably kidnapped, Lilah’s own curiosity and stubbornness get the better of her, and she finds herself swept into Con’s investigation.

He’s indifferent to propriety.
Con knows that Lilah hates him—he just wishes she weren’t so devastatingly beautiful, that he weren’t so attracted to her. Especially since they’re working closely together to solve the kidnapping, an adventure that leads them to Lilah’s peculiar childhood home, Barrow House, which sits atop an ancient fen and features an eerie maze on its grounds. They’ll have to join together to conquer a sinister force.

The more Con and Lilah uncover, the more they’re convinced that the answers lie buried deep within Barrow House—answers to a mystery darker than either of them could ever have realized.