The Septuagint is the Greek version of the Old Testament. It includes some books written in Greek which were never included in the Hebrew Bible, or the Bibles used by most Protestants. The books are called the Apocrypha (hidden, concealed). There are fifteen such books : 1 & 2 Esdras, Tobit, Judith, the Wisdom of Solomon, Ecclesiasticus, Baruch, the Epistle of Jeremiah, the Prayer of Azaariah, the Song of the Three Young Men, Susanna, Bel and the Dragon, the Prayer of Manasseh, 1 & 2 Maccabbees.
Of these the Roman Catholic Bible excludes 2 Esdras, which leaves the
14 books of the Apocrypha, which Roman Catholics, Anglicans, and some other
Protestant groups include in their lectionary readings. Because these books
were not in the original Hebrew canon (list of books) of the Old Testament
which are often quoted in the New Testament, we do not use them to establish
matters of doctrine. There are no quotations from the Apocrypha in the
New Testament, which perhaps suggests that the early Christians did not
use these books.