Charity Girl by Georgette Heyer
young and lovely runaway alone on the road to London Miss Charity Steane is running away from the drudgery of her aunt's household to find her grandfather. Not expecting her visit, the old gentleman is not in London but is away in the country. The first time the young Viscount Desford saw the girl, he thought she was a child. A child in trouble, running away. But she was a girl nearly seventeen and when Viscount encounters her walking to London alone, he feels honor bound to assist her. But after Desford rescued her, everyone assumed that he had run off with her. A scandal broth in the making, and dashing about the countryside together, the Viscount must prevent his exasperating charge from bringing him ruin upon herself... and him. Now in disgrace, Desford had to find the truth behind Cherry's strange behavior--before his own life came apart....
Who else should he turn to in such a scrape but his childhood playmate, Henrietta Silverdale? And although they refused to oblige their parents by marrying, they have always been the best of friends. But as Desford pursues Cherry's grandfather and father around the seedier fringes of society, Hetta is forced to wonder if he might not, at last, have fallen in love. Without the timely intervention of his brother and Hetta's suitor Gary Nethercott, Desford is in danger of making a jumble of his affairs.