I Will by Lisa Kleypas

Andrew, Lord Drake, has been cut out of his father’s will because of his dissolute manner of living. To be reinstated, Andrew decides to pretend that he has changed his wicked ways.

As part of his plan, he wants to convince his father that he is courting a respectable woman with the intention of marrying her. The problem is, he doesn’t know any decent women, except for his friend’s spinster sister, Miss Caroline Hargreaves. He blackmails the reluctant Caroline into helping him, and so the charade begins…