The Old Testament in Catholic Bibles contains seven more books than are found in Protestant Bibles (46 and 39, respectively). Protestants call them (inaccurately) the Apocrypha, while Catholics refer to them as the deuterocanon. These seven books are: Tobit, Judith, 1 and 2 Maccabees, Wisdom of Solomon, Ecclesiasticus (or, Sirach), and Baruch.
They were included in the Septuagint (the Greek translation of the Old Testament from the third century B.C.). This was the “Bible” of the apostles and Church fathers. St. Augustine, for instance, even regarded the Septuagint as inspired.
The Church councils at Hippo (393) and Carthage (397, 419) listed the deuterocanonical books (and the other 66 books) as Scripture, endorsing what had become the general belief of the universal Church. Pope Innocent I sanctioned the first two conciliar rulings in his Letter to Exsuperius in 405. The Council of Trent confirmed this canon in 1548.
The earliest Greek manuscripts of the Old Testament, such as Codex Sinaiticus (fourth century), and Codex Alexandrinus (c. 450) include these seven books mixed in with the others. The practice of collecting the into a separate unit dates back no further than 1520 (i.e., three years after the Protestant Revolt began). Thus, the separation of these books is the innovation or “novelty.”
Some have argued (I think with some force) that Martin Luther, the founder of Protestantism rejected the deuterocanon at least in part because he felt that they taught theological error. Luther freely granted that 2 Maccabees 12:46 taught the doctrine of purgatory, stating, “The text in Maccabees . . . is quite plain” (Letter to Georg Spalatin, 7 November 1519).
Yet Luther appears to contradict his “hostile” view of the deuterocanon in many places in his writings (even relatively late in his life), where he refers to several of its books as “Scripture”:
God confirms all this with many excellent examples in the Scriptures. . . . when Joseph and Azariah wanted to fight to gain honor for themselves, they were beaten [I Mac. 5:55-60]. (Whether Soldiers, Too, Can Be Saved, 1526)
The Holy Scriptures also praise such good will, as in Ecclus. 25:2 f.: . . . (Selected Psalms III, this citation is from December 1531)
Thus Scripture reports of the patriarch Jacob (Wisd. of Sol. 10:12): . . . (Commentaries on 1 Corinthians 7, 1 Corinthians 15 / Lectures on 1 Timothy; this citation is from 1 Corinthians 15, April 1533)
It is proper for the first book [of Maccabees] to be included among the sacred Scriptures, . . . (Preface to the Second Book of Maccabees, 1534)
The New Testament closely reflects the thought of the deuterocanonical books in many passages. For example:
Revelation 1:4 (RSV) . . . Grace to you and peace from him who is and who was and who is to come, and from the seven spirits who are before his throne,
Revelation 8:3-4 And another angel came and stood at the altar with a golden censer; and he was given much incense to mingle with the prayers of all the saints upon the golden altar before the throne;  and the smoke of the incense rose with the prayers of the saints from the hand of the angel before God.
Tobit 12:15 I am Raphael, one of the seven holy angels who present the prayers of the saints and enter into the presence of the glory of the Holy One.
St. Paul, in, seems to have one deuterocanonical passage very much in his mind, in the following statement:
1 Corinthians 15:29 Otherwise, what do people mean by being baptized on behalf of the dead? If the dead are not raised at all, why are people baptized on their behalf?
2 Maccabees 12:44-45 For if he were not expecting that those who had fallen would rise again, it would have been superfluous and foolish to pray for the dead.  But if he was looking to the splendid reward that is laid up for those who fall asleep in godliness, it was a holy and pious thought. Therefore he made atonement for the dead, that they might be delivered from their sin.
Likewise, we observe strong similarities in the two passages below:
Hebrews 11:35 Women received their dead by resurrection. Some were tortured, refusing to accept release, that they might rise again to a better life.
2 Maccabees 7:29 Do not fear this butcher, but prove worthy of your brothers. Accept death, so that in God's mercy I may get you back again with your brothers.
The deuterocanonical books are similar in style to other Old Testament books. Wisdom and Sirach are much like Proverbs. Tobit is somewhat in the same literary category as the book of Job; Judith is comparable to Esther (two heroic Hebrew women, who helped save their people); 1 and 2 Maccabees are historical narratives like the books of Kings and Chronicles, and Baruch is prophetic literature, akin to Jeremiah and other prophets like Isaiah. Baruch was Jeremiah’s secretary (Jer 36:4, 32).
St. Jerome was virtually (if not literally) alone among the Church fathers in his opposition to the deuterocanon as Scripture (hence is always prominently cited by Protestants in their arguments against it). Yet he included these books in his Vulgate translation, and like Luther, was inconsistent in his opinions.
He sometimes cited Sirach, Wisdom, and 1 and 2 Maccabees as Scripture. He referred (Letter 65, 1) to Judith as one of the virtuous women of the Bible, along with Ruth and Esther, and noted (in his Preface to Judith in the Vulgate) that the Council of Nicaea regarded the book as canonical.
Individual Church fathers are never binding authorities for Catholics, anyway. That is the role of the magisterium: popes and councils with the pope. Catholics have nothing to fear in the debate over the canonicity of these books. Church history is overwhelmingly on our side, as is internal evidence from the other 66 biblical books that all Christians agree are canonical.