Esther, Royal Beauty, Dangerous Beauty Series by Angela Hunt and published by Bethany House.
When Xerxes, king of Persia, issues a call for beautiful young women, Hadassah, a Jewish orphan living in Susa, is forcibly taken to the palace of the pagan ruler. After months of preparation, the girl known to the Persians as Esther wins the king's heart and a queen's crown. But because her situation is uncertain, she keeps her ethnic identity a secret until she learns that an evil and ambitious man has won the king's permission to exterminate all Jews--young and old, powerful and helpless. Purposely violating an ancient Persian law, she risks her life in order to save her people...and bind her husband's heart.