Esther is a part of the Biblical canon. You may be thinking of Judith, which is not part of the Biblical canon because it was written in Greek.

Setting the canon is the process by which each book in the bible was selected. A canon is a collection of works deemed authoritative and important. This means that books in the biblical canon are the most important surviving works on those subjects.

These are books contained in the Catholic canon but not recognized as biblical by the protestants. A list of the books are given in the "related links"

Canon comes from the Greek word kanon, meaning reed or measurement. A canonical book is one that measures up to the standard of Holy Scripture. The canon of Scripture refers to the books that are considered the authoritative Word of God.

The New Jerusalem Bible is a translation of the Bible that contains certain Old Testament books (as well as additions to Daniel and Esther) that are found in the Catholic biblical canon but not in the Protestant biblical canon. Thus, it would be accepted by Catholics but not by Protestants.