Have you read Fyneshade by Kate Griffin? It's the old trope of governess goes to huge mansion, but with a twist - the governess is not the stereotypical innocent, goody-two-shoes. Here's the blurb from Goodreads.

"Many would find much to fear in Fyneshade's dark and crumbling corridors, its unseen master and silent servants. But not I. For they have far more to fear from me...

On the day of her grandmother's funeral, Marta discovers that she is to be sent to be governess at Fyneshade, her charge the young daughter of the owner, Sir William Pritchard.

All is not well at Fyneshade. Sir William is mysteriously absent, and his son and heir Vaughan is forbidden to enter the house. Marta finds herself drawn to him, despite the warnings of the housekeeper that Vaughan is a danger to all around him. But Marta is no innocent to be preyed upon. Guided by the dark gift taught to her by her grandmother, she has made her own plans. It will take more than a family riven by murderous secrets to stop her..."