Origen (210 A.D.), "It should be stated that the canonical books, as the Hebrews have handed them down, are twenty-two; corresponding with the number of their letters." (Eusebius, Eccl. Hist.)

Hilary of Poitiers (360 A.D.), "The Law of the Old Testament is considered as divided into twenty-two books, so as to correspond to the number of letters." (Tractate on Psalms, prologue 15)

Athanasius (365 A.D.), "There are then of the Old Testament twenty-two books in number ... this is the number of the letters among the Hebrews." (Letter 39.4)
The original 22 Canonized OT was divided up into 3 parts.

TaNaKh
1) The Law (Torah)
2) The Prophets (Nevi'im)
3) The Holy Writings (Keturim)

Another common name for the third section would be "THE PSALMS" because Psalms was the book that headed off this "Royal Division."

Let me list the division and their books for you.

I. THE LAW (TORAH)
1) Genesis
2) Exodus
3) Leviticus
4) Numbers
5) Deuteronomy

II. THE PROPHETS
6) Joshua AND Judges
7) The Book of Kingdoms (Samuel and Kings)
8) Isaiah
9) Jeremiah
10) Ezekiel
11) The Twelve (Hosea to Malachi) 

III. THE HOLY WRITINGS (or THE PSALMS because it was the first book in the collection in this "Royal Division")
12) The Psalms
13) The Proverbs
14) Job
15) Song of Songs
16) Ruth
17) Lamentations
18) Ecclesiastes
19) Esther
20) Daniel
21) Ezra-Nehemiah 
22) The Book of Chronicles
Even Jesus Christ acknowledged the above books with their divisions as Scripture.
Luke 24:45 (NIV), "This is what I told you while I was still with you: Everything must be fulfilled that is written about me in the...
"Law of Moses ...
"The Prophets ... 
and ...
"The Psalms."