Captured By the Earl by Melanie Rose Clarke

As a member of the Distinguished Ladies of Purpose, Lady Emma Sinclair strives to help those less fortunate, even as others disregard her ideas as fanciful nonsense. That is, until she accidentally starts a rumor she is betrothed to Philip Fitzgerald, the powerful but absent Earl of Peyton. As society celebrates her victory in capturing the elusive lord, she enjoys new influence. Everything goes perfectly, until…

The earl returns.
Philip is shocked to arrive home to an unexpected betrothal. He doesn’t know Lady Emma’s motives or goals, yet the intelligent lady intrigues him. He will continue the ruse, chasing her secrets, even as he continues his own clandestine work. He saves people while in disguise, setting London aflame.
As everyone guesses the identity of the mysterious masked rescuer, Emma and Philip delve closer and closer to the truth, and each other.