The Assassin and the Libertine by Lily Riley — book slip

Daphne de Duras is a proper French duchess by day and a fledgling assassin by night.

Her latest mission is to execute Étienne de Noailles, a disgraced former noble and vampire emissary to King Louis XV's court, accused of murdering Madame de Pompadour.

However, Étienne's charm and the inconsistencies in the accusations lead Daphne to question his guilt.

As they form an uneasy alliance to uncover the real killer, Daphne finds herself drawn to the enigmatic vampire, challenging her deep-seated loathing for his kind.

First in the 'Les Dames Dangereuses' series.