The genre of the book of Esther is Narrative History. Its author is anonymous however; some believe Mordecai, (Esther's cousin and guardian), wrote it. It was written approximately 470 B.C. in Persia. Esther became queen in 479 B.C. The key personalities are Esther, Mordecai, King Ahasuerus (or Xerxes), and Haman.
Its purpose is to demonstrate God's love and sovereignty in all circumstances. It is a post-exile story about Jews who stayed behind after most returned to Jerusalem after captivity. Babylon was conquered by Persia and Esther miraculously becomes the queen of the land, and saves her people.