The Devil On Horseback - Victoria Holt
Synopsis:

From the moment that beautiful eighteen-year-old Minella Maddox saw Charles-Augueste, the haughty, arrogant Comte Fontaine Delibes, his satanic looks thrilled her with a sense of danger and excitement. The Comte, too, liked what he saw. He decided that he would marry Minella, and nothing would be allowed to stand in his way . . . not even his wife
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I just finished this and I'm exhausted. This one had me tied in knots with all the twists and turns.

The heroine of the book is the English daughter of the local schoolmarm who comes to the attention of the dark, and married, French Comte. When her mother dies, the school fails and she finds herself the companion of the H's daughter, who has her own set of aggravating issues.

The story is set during the time of the French Revolution, so there was that angle to add to the suspense. There were many conspiracies and dastardly deeds and everyone [illegimate sons, adoptees, and OW (not just the wife)] was a suspect up until the very end, which was a serious nail-biter for me.

I noticed quite a few scathing reviews from those who, I imagine, didn't appreciate a married hero, so this might prove to be a dealbreaker for those who can't abide by cheating whoredogs. But if you like dark and dangerous anti-heroes, heroines that aren't exactly pushovers, and lots of seat-pinching excitement, then you might enjoy this one.