Eliza and the Duke by Harper St. George

American heiress Eliza Dove was resigned to a polite marriage of convenience…until she spent one wicked night with the Duke.
All hopeless romantic Eliza Dove asked for was one night of adventure. One glorious evening of freedom to explore the dark corners of London with a mysterious stranger before a lifetime trapped in a quiet, respectable marriage of convenience. Except now she wants more. Now she wants him.
Simon Cavell is no gentleman. Known only as ‘the Duke,’ Whitechapel’s prize boxer is one fight away from achieving his goal: to safeguard his late sister’s only treasure and leave the streets for good. He cannot allow some pretty young heiress to spill his secrets, no matter how tempting she might be. In return for her silence, Simon will give Eliza a taste of the darkness…and hope he doesn’t lose his heart in the process.
But one night together could never be enough. And now Eliza has a new plan—an even more scandalous bargain that will either land the heiress her duke or ruin them both.