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The Countess Conspiracy by Courtney Milan
Spoiler:
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.
The Countess Conspiracy is the third full-length historical romance in the Brothers Sinister series. The other books in the Brothers Sinister series are:
½. The Governess Affair (prequel novella)
1. The Duchess War
1½. A Kiss for Midwinter (a companion novella to The Duchess War)
2. The Heiress Effect
3. The Countess Conspiracy
4. The Suffragette Scandal
4½. Talk Sweetly to Me (a companion novella to The Suffragette Scandal)