Duel of Shadows by B. A. Lovejoy

The dead owe the living no favors.

This I knew well, considering the fact that my father himself had told it to me before he passed, leaving my lavish lifestyle locked in limbo and dependent on one thing and one thing only: the fulfillment of a lifelong promise, my father’s bloodline merging with the Duke of Bedford’s, the man responsible for our great fortune.
I had been betrothed to a man I barely knew from the moment I was born. A man who despite my father’s passing, decided to take another to his bed.
To receive my family fortune, however, I must marry a Douglas, and I am surprised to find that the current Duke of Bedford is not the only Douglas brother in London—his younger brother, James, has become a mortician— the most renowned mortician in London.
But James was supposed to have passed from the rattling cough years before, and he refuses to tell me how he survived, or what his connection is to the mysterious pleasure house owner, Nicholas Francis.
Just when the unearthly mystery can’t get any deeper, and magic seems to be the only solution, Archibald Ross, who dropped before me on his knee, with no care for my family fortune arrives in London and— he’s brought two detectives in tow.

Two detectives who are aware of James’ deadly connection to Nicholas Francis.