Jerome translated the Twelve Prophets (i.e., “Minor Prophets”) around AD 394. As was his custom, he prefixed this brief note to his translation, dedicating it to two of his friends (Paula and Eustochium).
The order of the Twelve Prophets is not the same among the Hebrews as it is among us. Hence, according to that which is read there [in the Hebrew], here [in my version] also they are set down. Hosea is broken up into clauses, and speaks as if in aphorisms. Joel is clear at the beginning, quite obscure at the end. And the individual [prophets] have their own characteristics all the way to Malachi, which the Hebrews assert to be Ezra, the scribe and teacher of the law. And because it would take too long now to speak concerning all of them, this alone, O Paula and Eustochium, I wish you to take to heart, that the Twelve Prophets are one book, and Hosea is a contemporary of Isaiah, but Malachi lived in the times of Haggai and Zechariah. But when no date is given in the title, that prophet prophesied under the same kings as the preceding prophet that does have a date in the title.
The old NPNF translation of Jerome's prefaces does not contain this preface, but merely the following brief note:
This Preface, dedicated to Paula and Eustochium in A.D. 392, contains nothing of importance, merely mentioning the dates of a few of the prophets, and the fact that the Twelve Prophets were counted by the Hebrews as forming a single book.
On the contrary, I find this preface to be a helpful introduction to the reception of the Minor Prophets among early Christians. Jerome crams a lot of interesting tidbits into this brief introduction.
First, he names them the Twelve Prophets, rather than the title more traditional for Western Christians, viz., “Minor Prophets”. This is because this latter title was apparently not yet coined by the time Jerome published his translation. The earliest attestation for the designation “Minor Prophets” is found in Augustine’s City of God, book 18, published in the early to mid-420s. Before Augustine, everyone called these books “The Twelve”, going back as early as Ben Sira (49:10) in the early second century BC. This is the unanimous testimony of the early Jewish and Christian canonical lists, and is still the custom among Jews.
These same sources also provide testimony for the custom of counting the Twelve Prophets as one book, a matter stressed by Jerome in this preface. Again, Augustine seems to be the first writer to count the Twelve as twelve instead of as one, and his numbering system became standard in western Christianity. All Christians before Augustine, and Jews up to the present day, counted the Twelve as a single book.
One thing further on this point: it is interesting that Jerome is so insistent in this preface that the Twelve count as one book, and in his translation he does not provide prologues for each prophet. In other words, he treats them as one book in his translation. However, in his commentary on the Twelve, which he would begin to publish shortly after this translation, and which would be complete in 406, he does not at all treat them as one book. There, he offers no general preface for the entire corpus, but instead writes introductions for each prophet separately. In this, he does not differ from his contemporaries or predecessors. Despite this preface, Jerome was not, after all, a forerunner of the recent scholarly attempts to read the Twelve as a single book.
The second point of interest in this preface is that Jerome points out the divergence in order of the Twelve Prophets between the Septuagint (LXX) and Jewish tradition. We are familiar with the Hebrew order, thanks to Jerome’s reliance on the Hebraica veritas (“Hebrew truth”) in his Vulgate translation of the OT. The LXX differs from it in the first half of the Twelve, giving the following order: Hosea, Amos, Micah, Joel, Obadiah, Jonah. The order for the rest of the books is identical.
It is unclear how early the internal order of the Twelve was fixed in the Hebrew tradition, though many scholars assume that Ben Sira’s reference to the twelve prophets (49:10), and most of the evidence from the Dead Sea Scrolls, confirms that it was fixed by the early second century BC. It is equally unclear what principles governed the arrangement of the books, though they are assembled roughly chronologically. The books that don’t fit this paradigm are Joel and Obadiah, though they do not explicitly indicate a time period and are notoriously difficult to date, so they may in fact fit this schema, at least in the mind of their ancient editor.
It is generally assumed that the LXX order was determined by the length of the books, so that Joel and Obadiah follow the longer books of Hosea, Amos, and Micah. Jonah is set by itself because it is unique in not offering prophetic oracles, but instead a story about a prophet. Why these principles were not carried through the entire corpus, but only affected the first half, is, again, unclear. The reader can judge for himself how convincing is this line of reasoning, and he is also invited to propose a better solution.
These comments on Jerome’s preface to the Twelve cover only the first sentence. Perhaps in the future I will have something to say about the rest.
 The translation of this preface by Kevin P. Edgecomb may be viewed at his site or here. I offer in the present post my own translation. The differences are mostly stylistic.
 This phrase is translated thus by A.A. Macintosh, Hosea, ICC (Edinburgh: T.&T. Clark, 1997), p. lxiv. Macintosh considers Jerome’s evaluation of Hosea’s style “entirely apposite”.
 This is a loose translation of Jerome’s much more complicated syntax. A more literal translation runs as follows: “But in those for which the time is not displayed in the title, they prophesied under those kings under whom also they prophesied who have titles before them.”
 The first sentence of ch. 29 runs as follows in the NPNF translation: “The prophecy of Isaiah is not in the book of the twelve prophets, who are called the minor from the brevity of their writings, as compared with those who are called the greater prophets because they published larger volumes.”
 See the lists provided by H.B. Swete, An Introduction to the Old Testament in Greek2 (1914), pp. 198ff.
 See On Christian Doctrine 2.13 . Augustine does say that the Twelve are counted as one, but later in the same paragraph he asserts that the books of the OT total 44, a number which assumes that the Twelve count as twelve. Augustine also includes the Apocrypha, of course.