1) Canonical Shaping of the Twelve:
It is "clear that the placements of later books next to earlier ones is an intentional move, arising from the canonical process itself, and is not a reader-response imposition by readers tired of older approaches and looking for new ones. Just as YHWH's roaring from Zion ends Joel and begins Amos, Amos ends with a promise of Edom's (9:12), and Obadiah unhesitatingly describes it. Jonah provides an occasion not of Israelite but of Ninevite repentance, which makes the prophet sore but which reminds the reader that God is not above relenting over evil powers like Edom (whom he has punished in Obadiah already) or even the powerful nation of Assyria. He can treat them with the same patience and kindness he has lavished on his own people, in different ways and dispensations, in Hosea, Joel, and Amos or in the context of Edom's destruction of Obadiah (17-21). Micah establishes the limits of God's patience, now toward the preserved remnant of Judah, strikingly at the exact middle point of the Twelve as a whole (3:12)—a prophecy that bore repeating in a later conflict over Jeremiah's similar preaching against the temple and king (see Jer. 26:18)" (p. 237).
2) The nature of the phenomenon:
"We know that as a historical datum, very soon after the final prophetic book (Malachi?) took shape, the Twelve are regarded as a collection (Sirach, Qumran). Indeed, in Sirach the existence of the Twelve is something of a cliché: it is referred to as a given, without argument or assertion. The literary evidence for internal editorial affiliation, linkages, and intentional juxtapositions is becoming increasingly clear as scholars turn to this sort of inquiry. And I have tried to probe into the theological coherence of the twelve-book collection that may be the intended consequence or, indeed, the originating engine, driving the historical and literary dimensions of canonical shaping "(217, emphasis original).