The Guardian — Mary Ann Gibbs

At seventeen, Sally Lofthouse was just past the schoolgirl stage, deeply infatuated with her neighbor, Edward Martin, and busily attending parties. But her friend, Emma Fitzjames, led a totally different life. Emma's destiny was controlled by an all-powerful and distant guardian whom no one had ever seen.

In a way, their friendship was the undoing of them all -- or the secret of Emma's guardian would still be hidden. But Sally's father, with schemes of his own, used Emma's plight to his own advantage. And as for Edward Martin, he chose Emma over Sally, and then went in search of the guardian for permission to marry....