The Book of Esther is a book of the Bible Old Testament and Jewish Tanakh.
The Biblical Book of Esther is set in the time of Xerxes I (in Greek; Persian: Khshayarsha; Hebrew: Ahasuerus), the Persian king (486 - 465 B.C) who continued his predecessors' war against the Greeks; in 480 B.C the Greeks defeated his navy at the Battle of Salamis, and in 479 B.C they defeated his army at the Battle of Platea, and then Xerxes gave up on trying to conquer Greece, and that was the beginning of the end for the Persian Empire as the top power in the Middle East. Esther was written several generations later, possibly as late as 130 B.C, after the historical facts had become somewhat garbled in people's minds.
In the story, Xerxes is married to Vashti, whom he puts aside (484 B.C). Mordecai's cousin Hadassah is selected from the candidates to be Xerxes's new wife and assumes the "throne name" of Esther (480 B.C). His prime minister Haman (an Amalekite) and Haman's wife Zeresh plot to have Xerxes kill all the Jews without knowing that Esther is Jewish. Esther saves the day for her people: Haman is hanged on the gallows he had had built for Mordecai, and Mordecai becomes prime minister in Haman's place.
Mordecai is described as having been carried away from Jerusalem with Jeconiah by Nebuchadnezzar, which happened in 597 B.C, some 110 years before. Xerxes's wife in the late 480s was Amestris, a daughter of one of his generals. The Elamites had been a people whose capital city was Shushan and whose traditional enemy was Babylon. Elam had been crushed (in 640 B.C) by the Assyrians, whose campaign against the Elamites drew the Assyrian forces away from Judah, thus saving the Jews; at the same time, the Assyrians were so weakened by their efforts that Assyria fell to Persia, and Darius I built Susa where Shushan had been. By the time Esther was written, the foreign power visible on the horizon as a future threat to Judah was the Macedonians of Alexander the Great, who defeated the Persian empire about 150 years after the time of the story of Esther, which is why it has Xerxes call Haman a Macedonian instead of an Amalekite, who had been the Jews' enemies at an earlier time.
"Vashti" was the name of the chief Elamite goddess. "Esther" is Aramaic for "Ishtar", the chief Babylonian goddess, and "Hadassah" comes from the Babylonian for "bride" and was one of Ishtar's titles. "Mordecai" is Hebrew for "Marduk", the Babylonians' chief god. "Haman" comes from "Hamman", the name of the Elamites' chief god, and "Zaresh" is similar to the name of Hamman's goddess-consort "Kirisha". Thus, the Book of Esther must be read as an allegory for Babylonia's overcoming Elam -- so the Babylonian gods replaced the Elamite gods in Shushan -- expressing the spirit of the time when it was written, when the Jews were an independent kingdom again after generations of bitter persecution.
Esther is the only book of the Bible that does not mention God; later writers tried to remedy that perceived lack by making additions, but those additions were relegated to the Apocrypha when the King James Version was compiled. Although he recognized them as later additions, Jerome included them in his compilation, so they appear in Catholic Bibles.