My Lady Rogue (Nelson's Tea #2.5)
Katherine Bone

Lord Simon Danbury’s loyalty to the crown has never been questioned — until now. As death’s darkening veil cascades over London, a hostile mole inside Nelson’s Tea tries to assassinate him. Surrounded by the greatest spies in England, only one thing stands to defeat him — losing the one woman who has made life worth living.
Everything Simon and Gillian have done has led to this moment… Will it be too late?