Who wrote the book of Esther and for what purpose?

It is believed that Mordecai (Esther's cousin) wrote Esther. The author's central purpose was to record the institution of the annual festival of Purim and to keep alive for later generations the memory of the great deliverance of the Jewish people during the reign of King Xerxes (a.k.a. King Ahasuerus). The book accounts for both the initiation of that observation and the obligation for its perpetual commemoration.