The Earl Who Knew Too Much by Monica La Porta

Falling in Love with the Enemy has never been so sinful…

After years of mourning an empty grave, the Earl of Hawkshead can finally put his brother to rest, and the French spy responsible for Alexander’s death is about to be apprehended. Justice is finally within reach, but Hawk is more interested in pursuing the mysterious Lady Cooper, who has awakened his senses and made him forget about his responsibilities.
Delphine is desperately trying to save her siblings and has fallen prey to the ruthless Mistral. Forced to infiltrate a house party to garner information for the spy, the last thing she expects is to develop feelings for the forbidding Hawkshead—a man who can easily ruin her and destroy her family.
Hawk soon discovers there is more to Lady Cooper than meets the eye and that she might be working with the very spy he wants to see hang. Will his growing attraction for her win over his familial duty to avenge his brother? Or will he denounce her as Mistral’s accomplice?