The protocanonical books are those books of the Old Testament which were coextensive with the Hebrew Bible and which have always been considered canonical by almost all Christians throughout history. The term protocanonical is often used to contrast these books to the deuterocanonical books or apocrypha, which "were sometimes doubted" in the early church.
These books are typically 39 in number in most English bibles. Based on the Jewish tradition of the Tanakh, these same books may be counted as 24 books, counting the twelve minor prophets together as one book, one book each for 1 and 2 Samuel, 1 and 2 Kings, and 1 and 2 Chronicles, as well as a single book for Ezra and Nehemiah. In his prologues, Jerome  counted the same content as 22 books, combining Jeremiah with Lamentations and Judges with Ruth. The list given in Codex Hierosolymitanus numbers the same books at 27.
These enumerations were sometimes given a numerological significance. The 22-book enumeration was said to represent the number of letters in the Hebrew alphabet; the 5 double books (Judges/Ruth, 1/2 Samuel, 1/2 Kings, 1/2 Chronicles, Ezra/Nehemiah, and Jeremiah/Lamentations) representing the five Hebrew letters that have double forms, chaph, mem, nun, phe, and sade. The 24-book enumeration was said to be represented by the 24 elders who throw their crowns before the Lamb in the Book of Revelation. The 27-book enumeration balances one-for-one the 27 canonical books of the New Testament.
The protocanonical books were widely but not universally accepted by Christians as being in the canon. Athanasius omitted Esther  from his list. He may have been following an early 22-book Jewish canon, possibly the same one mentioned but not specified by Josephus. Theodore of Mopsuestia omitted Song of Songs, Ecclesiastes, Job, and Ezra-Nehemiah to obtain a listing of 22 books. Most famously, Marcion rejected all of the protocanonical books, preferring a Bible consisting only of a restricted New Testament.
The list of protocanonical books is Genesis, Exodus, Leviticus, Numbers, Deuteronomy, Joshua, Judges, Ruth, 1–2 Samuel, 1–2 Kings, 1–2 Chronicles, Ezra, Nehemiah, Esther, Job, Psalms, Proverbs, Ecclesiastes, Song of Solomon, Isaiah, Jeremiah, Lamentations, Ezekiel, Daniel, Hosea, Joel, Amos, Obadiah, Jonah, Micah, Nahum, Habakkuk, Zephaniah, Haggai, Zechariah, and Malachi.
By analogy with the early and broad acceptance of the Hebrew scriptural texts, the term protocanonical is also sometimes used to describe those books of the New Testament which were more widely accepted by the early Church than some of the other 27 books recognized today by almost all Christians. For more information concerning the development of the New Testament canon, see the article Biblical canon.
- Old Testament of Douay, Vol. 1, Proemial Annotations, 1635
- Prologus Galeatus, English translation
- published by J.-P. Audet in JTS 1950, v1, pp. 135–154, cited in The Council of Jamnia and the Old Testament Canon, Robert C. Newman, 1983.
- "The Old Testament of the Early Church" Revisited, Albert C. Sundberg, Jr., 1997.
- Prologues of St. Jerome, Latin text
- Letter 39, Athanasius of Alexandria, English translation
- Theodore of Mopsuestia, article from the Catholic Encyclopedia