Persuasions of an Earl’s Daughter by Samantha Holt

She's not about to let him steal anything else from her...least of all her heart... Lady Lilly Musgrave is strong, capable, and utterly unconcerned with what society thinks of her. So, when her newly inherited horse is stolen, hunting down the culprits seems only natural. Her first suspect is obviously the man who wanted the horse more than she did—an arrogant, unfairly handsome rake with an even more scandalous reputation than her own... Lord August Beresford is a great many things. But he's most certainly not a thief. He should let her go off on her fool's errand alone...but he can't. So, he'll do whatever is necessary to protect her while she's investigating the crime. He will not, however, do anything foolish along the way...like fall for the prickly beauty... It's not long before Lilly realizes there's more to August than meets the eye—and she likes everything she's seeing. Unfortunately, persuading him to trust her with his dark secrets won't be easy. Good thing she was never one to walk away from a challenge...or love...