Never Woo the Wrong Lady by Charlie Lane

To protect his guardianship of his beloved younger brother, newspaperman Tristan Kingston turns to courtship and marriage. But the woman must be just right—maternal, proper, and unbothered by marrying an earl’s bastard. Dreamy yet practical Lady Andromeda Merriweather seems like the perfect choice.
But she’s already taken.
Everyone thinks Lady Andromeda is engaged to a baron in Cornwall. She’s not. It’s a ruse to hide a bigger secret: she runs an erotic lending library with her four sisters. When the confident Mr. Kingston swaggers in, announcing his intentions to court her, Andromeda’s in trouble. In every way possible.
Because she’s got a secret, and he’s a newspaperman.
Because he wants to marry her, and she’s fake engaged.
Because he kisses her, and she wants more.
He doesn’t seem to care she’s unavailable. He doesn’t seem to believe it. And while his attentions threaten her secret, he’s a much bigger threat to her heart.