The Mysterious Key and What It Opened

First published in 1867, "The Mysterious Key and What It Opened" is a tale of mystery and romance by Louisa May Alcott. 

The story follows Paul, an orphaned boy and Lillian Trevlyn, beautiful and rich young girl. 
When Lillian Trevlyn is a baby, her father dies under mysterious circumstances, and she nearly loses her mother on the very same day. Lillian grows up to become a lovely young woman, but the mysteries of her childhood remain unsolved.
Paul, an orphaned boy, enters the story as a groomsman. Though he is warmly received by the family he is furtive and evasive regarding his past and future plans. The romantic mystery of the Trevlyn family catches full fire when Paul disappears.