The Countess Conspiracy by Courtney Milan (Brothers Sinister #3)
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Sebastian Malheur is the most dangerous sort of rake: an educated one. When he's not scandalizing ladies in the bedchamber, he's outraging proper society with his scientific theories. He's desired, reviled, acclaimed, and despised-and he laughs through it all.
Violet Waterfield, the widowed Countess of Cambury, on the other hand, is entirely respectable, and she'd like to stay that way. But Violet has a secret that is beyond ruinous, one that ties her irrevocably to England's most infamous scoundrel: Sebastian's theories aren't his. They're hers.
So when Sebastian threatens to dissolve their years-long conspiracy, she'll do anything to save their partnership...even if it means opening her vulnerable heart to the rake who could destroy it for good.