Rather than me telling you, why don't we go straight through the horse's mouth? :D
There are twenty-two letters among the Hebrews, as is also witnessed by the language of the Syrians and Chaldeans, which is for the most part similar to the Hebrew; for these twenty-two elements also have the same sound, but different characters. The Samaritans still write the Pentateuch of Moses in the same number of letters, only they differ in shapes and points (or "endings" apicibus). And Ezra, the scribe and doctor of the Law, after the capture of Jerusalem and the rebuilding of the Temple under Zerubbabel, is certain to have found (or "invented" repperisse) other letters, which we now use, when up to that time the characters of the Samaritans and the Hebrews were the same. In the book of Numbers this same total is also mystically shown by the census of the Levites and the priests. And we find in certain Greek scrolls to this day the four-lettered Name of God written in the ancient letters. But also the thirty-sixth Psalm, and the one hundred tenth, and the one hundred eleventh, and the one hundred eighteenth, and the one hundred forty-fourth, although written in different meter, are nevertheless woven with an alphabet of the same number. And in the Lamentations of Jeremiah, and his prayer, also at the end of the Proverbs of Solomon from that place in which he says "Who can find a strong woman?" are counted the same alphabet or sections. Furthermore, five of the letters among them are double: chaph, mem, nun, phe, sade. For they write with these one way at the beginning and in the middle of words, another at the end. From which also five are considered double books by most: Samuel, Malachim, Dabreiamin, Ezra, Jeremiah with Cinoth, that is, his Lamentantion. Therefore, just as there are twenty-two elements, by which we write in Hebrew all that we say, and the human voice is understood by their beginnings (or "parts" initiis), thus twenty-two scrolls are counted, by which letters and writings a just man is instructed in the doctrine of God, as though in tender infancy and still nursing.
The first book is called among them Bresith, which we call Genesis; the second, Hellesmoth, which is named Exodus; the third, Vaiecra, that is Leviticus; the fourth Vaiedabber, which we call Numbers; the fifth, Addebarim, which is designated Deuteronomy. These are the five books of Moses, which they appropropriately call Thorat, that is, the Law.
The second order is made of the Prophets, and begins with Jesus son of Nave, which is called among them Joshua benNum. Then they append Sopthim, that is the book of Judges; and they attach Ruth to the same, because the history narrated happened in the days of the Judges. Samuel follows third, which we call First and Second Kingdoms. Fourth is Malachim, that is Kings, which book contains Third and Fourth Kingdoms; and it is much better to say Malachim, that is Kings, rather than Malachoth, that is Kingdoms, for it does not describe the kingdoms of many nations, but only that of the Israelite people which contains twelve tribes. Fifth is Isaiah, sixth Jeremiah, seventh Ezekiel, eighth the book of the Twelve Prophets, which is called Thareasra among them.
The third order holds the Hagiographa, and begins with Job, the first book, the second by David, which is also one book of Psalms comprising five sections. The third is Solomon, having three books: Proverbs, which they call Parables, that is Masaloth, and Ecclesiastes, that is Accoeleth, and The Song of Songs, which they denote with the title Sirassirim. Sixth is Daniel, seventh Dabreiamin, that is Words of the Days, which we may call more clearly a chronicle (Gk here: χρονικον) of all of Divine history, which book is written among us as First and Second Paralipomenon; eighth is Ezra, which is also in the same manner among Greeks and Latins divided into two books; ninth is Esther.
And thus there are likewise twenty-two books in the Old (Testament), that is five of Moses, eight of the Prophets, nine of the Hagiographa. Although some may write Ruth and Cinoth among the Hagiographa, and think of counting these books among their number, and then by this to have twenty-four books of the Old Law, which the Apocalypse of John introduces with the number of twenty-four elders worshipping the Lamb and offering their crowns, prostrated on their faces, and crying out with unwearying voice: "Holy, holy, holy Lord God almighty, Who was and Who is, and Who will be."