The Lost Jewel — A. Nation

This expanded prequel to Mystery Along the Danube is about life and marriage in the fifteenth century.
Count Gawain loses his wife and has to raise his only heir and daughter. When she comes of marriageable age, she wants to run off with her lover. When the count dies, Katheryn is forced to marry the viceroy. Now, how will he win her favor and still keep his secret?