The Talmud describes Esther writing to the sages and requesting that they write me for the generations, that is to say, immortalise her story by including it in the canon of the Bible (Megillah 7a). The same source discusses attempts to prove that the Book of Esther was written through divine influence and is therefore worth including in the Bible and was not a secular story of political intrigue as it might first appear. The Talmud also records debates about the inclusion of Kohelet (Ecclesiastes) and Song of Songs in the Bible.
These passages show that there was still some fluidity about the precise form of the Tanach, the Hebrew Bible, in rabbinic times. Tanach is an acronym for Torah, Neviim and Ketuvim, the Five Books of Moses, the Prophets and the Writings (comprising Psalms, Proverbs, the megillot etc). While the Torah and Neviim were fixed much earlier, the Ketuvim were still being argued over in the Talmud.
Traditionally, there are 24 books of the Tanach, though English Bibles usually come up with 39 books by adding subdivisions. Christians called the Tanach the Old Testament, but that term should be non-PC today, implying as it does the necessary augmentation of the old by the new. Through canonisation of the Tanach, the Jews became the people of the Book, and the Book became the animating force of Jewish existence.