Dalliance with the Duchess by Sara Bennett

Far from being enemies, Sophia and Nicholas discover they are ideally matched. Can they find a happy ever after?

Ten years after marrying the Duke of Oldney, Sophia is a widow, living quietly without the society intrigue and gossip she once craved. But when the man she hates, Nicholas Blake, asks for her help to save his young friend from Oldney’s vicious friends, Sophia agrees to return to a world she now despises. Nicholas has only ever been her adversary, but as they work closely together, loathing turns to irresistible desire. Could she have been wrong about him?
Nicholas Blake has always found Sophia fascinating, even when they were sworn enemies, but suddenly blazing anger has turned into unstoppable passion. How can he focus on important matters like saving his friend and finding his missing sister, when all he can think about is Sophia?