The Catholic Bible has 73 books, while the Protestant Bible has 66 book. Why?
Because we use the same Bible that Jesus used with only the addition of the New Testament. The Jews of Christ's day did not recognize the Apocrypha as being a part of the Holy Scriptures.
Writing a rebuttal to anti-Jewish propaganda in the latter part of the first century, Josephus describes the Hebrew canon of scripture which was recognized by the Jews.
"For we have not an innumerable multitude of books among us, disagreeing from and contradicting one another , but only twenty-two books, which contain the records of all the past times; which are justly believed to be divine... (Contra Apion 1:8).
The same 39 books that we have in our Bible were condensed into the 22 books of the Hebrew Bible. For example, they had a single book of Samuel and of Kings and of Chronicles. The Minor Prophets were grouped together into a single book called the Twelve.
"It is true, our history has been written since Artaxerxes, very peculiarly, but has not been esteemed of the like authority with the former by our forefathers, because there has not been an exact succession of prophets since that time." (Contra Apion 1:8).
Josephus tells us that the Jews rejected the Apocrypha because it had not been penned by a prophet and because there had been no line of prophets who spoke and who wrote the words of God.
Also Athanasius penned an official list of books that were to be included in the canon in his 39th Festal letter (367 AD), in it he listed all of the books in the Protestant canon as truly canonical and some of the books we know of as the Apocrypha he listed because he thought they were "appointed by the fathers" - here are the relevent paragraphs from the letter:
4. There are, then, of the Old Testament, twenty-two books in number; for, as I have heard, it is handed down that this is the number of the letters among the Hebrews; their respective order and names being as follows. The first is Genesis, then Exodus, next Leviticus, after that Numbers, and then Deuteronomy. Following these there is Joshua, the son of Nun, then Judges, then Ruth. And again, after these four books of Kings, the first and second being reckoned as one book, and so likewise the third and fourth as one book. And again, the first and second of the Chronicles are reckoned as one book. Again Ezra, the first and second are similarly one book. After these there is the book of Psalms, then the Proverbs, next Ecclesiastes, and the Song of Songs. Job follows, then the Prophets, the twelve being reckoned as one book. Then Isaiah, one book, then Jeremiah with Baruch, Lamentations, and the epistle, one book; afterwards, Ezekiel and Daniel, each one book. Thus far constitutes the Old Testament.
5. Again it is not tedious to speak of the books of the New Testament. These are, the four Gospels, according to Matthew, Mark, Luke, and John. Afterwards, the Acts of the Apostles and Epistles (called Catholic), seven, viz. of James, one; of Peter, two; of John, three; after these, one of Jude. In addition, there are fourteen Epistles of Paul, written in this order. The first, to the Romans; then two to the Corinthians; after these, to the Galatians; next, to the Ephesians; then to the Philippians; then to the Colossians; after these, two to the Thessalonians, and that to the Hebrews; and again, two to Timothy; one to Titus; and lastly, that to Philemon. And besides, the Revelation of John.
6. These are fountains of salvation, that they who thirst may be satisfied with the living words they contain. In these alone is proclaimed the doctrine of godliness. Let no man add to these, neither let him take ought from these. For concerning these the Lord put to shame the Sadducees, and said, `Ye do err, not knowing the Scriptures.' And He reproved the Jews, saying, `Search the Scriptures, for these are they that testify of Me.'
7. But for greater exactness I add this also, writing of necessity; that there are other books besides these not indeed included in the Canon, but appointed by the Fathers to be read by those who newly join us, and who wish for instruction in the word of godliness. The Wisdom of Solomon, and the Wisdom of Sirach, and Esther, and Judith, and Tobit, and that which is called the Teaching of the Apostles, and the Shepherd. But the former, my brethren, are included in the Canon, the latter being merely read; nor is there in any place a mention of apocryphal writings. But they are an invention of heretics, who write them when they choose, bestowing upon them their approbation, and assigning to them a date, that so, using them as ancient writings, they may find occasion to lead astray the simple.
This canon was ratified at the council of Carthage (I may be wrong about that). Somehow the ones mentioned in paragraph seven became "deuterocanonical" or a "secondary canon", though initially they were no more than useful for "instruction in godliness". Interestingly his list also included the "Teaching of the Apostles" (The Didache) and the "Shepherd of Hermas" neither of which became part of the apocrypha/deuterocanon and were lost from Christian Bibles quite early. You'll note also that Baruch is listed as canonical and part of Jeremiah and that Esther was considered apocryphal in the 330's.
Hope this helps and thanks for bringing up the issue.