First Comes Scandal : Book 4 by Julia Quinn

She was given two choices...
 
Georgiana Bridgerton isn't against the idea of marriage. She'd just thought she would have some say in the matter. But with her reputation hanging by a thread, Georgie is given two options : live out her life as a spinster or marry the rogue who has ruined her lfe. 
 
Enter option three.
 
As the fourth son of an earl, Nicholas Rokesby is prepared to chart his own course. He has no time - or interest- in finding a wife. But when he discovers that Georgie Bridgerton - his literal girl-next-door - is facing ruin, he knows what he must do. 
 
A marriage of convenience.
 
It might not have been the most romantic of proposals, but Nicholas never thought she'd say no. Georgie doesn't want to be anyone's sacrifice and besides, they could never think of each other as anything more than childhood friends...could they?