If you trace the history of a text from earlier manuscripts to later ones, it’s not unusual for the text to be extended in various ways. If you look at the life of a manuscript over time, it’s not unusual for additional leaves or quires to be added. Or if you look at the edition history of early printed books, it’s not unusual for printers to add value by supplementing a text with additions of various kinds. These processes of accretion are common enough in the material I’m familiar with that I suspect they’re widespread in history. When I read the Book of Mormon, I think I see the result of similar processes, both preceding Mormon’s own compilatory work and exemplified by it.
Specifically, I think the Book of Mormon consists of three fundamental narrative cores: 1 Nephi, Mosiah-3 Nephi 7, and Mormon’s own prophetic book. (If you prefer to draw the boundaries at 2 Nephi 5 and Helaman, I won’t argue.) I would guess that compilers prior to Mormon created the first two of these core historical narratives from a variety of earlier texts, including the psalms, sermons, speeches, dialogues, and epistles that are still distinctly visible in them. The compiler of 1 Nephi is usually identified with the person of Nephi. The compiler of Mosiah-3 Nephi 7 is usually identified with Mormon. I suspect hands beyond those identified in the text helped shape the books of 1 Nephi-Omni, however, and that Mormon was not the first to assemble Mosiah-3 Nephi. (Gary Sturgess has argued that Alma the Younger or a disciple of his was the compiler of Mosiah.)
Each of the three narrative cores,1 Nephi, Mosiah-3 Nephi 7, and Mormon, has in turn been supplemented with additional material that bears some relation to the original core.
- The narrative of 1 Nephi was extended with 2 Nephi, consisting of Lehi’s blessings, Jacob’s sermons, extracts from Isaiah, and Nephi’s prophecies. Others, including Gardner, have noted the different character of the two books, although some see the boundary as 2 Nephi 5/6.
- The historical account from Mosiah to 3 Nephi 7 was extended by 3 Nephi 8-28:16.
- Mormon’s own book, an account of the destruction of the Nephites, was extended with the book of Ether, an account of the destruction of the Jaredites.
I think the first two of these extensions both preceded Mormon, although he likely had some role in the second, while the third extension would be the work of Moroni.
Each of these extended compilations in turn have gained additional minor related texts.
- 1-2 Nephi gained small, topically related books in the form of Jacob-Jarom-Omni. (Mormon seems to describe 1 Nephi-Omni as a composite work: a “small account of the prophets, from Jacob down to the reign of this king Benjamin, and also many of the words of Nephi”; interestingly, this reverses the order found in the Book of Mormon today.)
- Mosiah-3 Nephi 28:16 gained 4 Nephi.
- The dual accounts of national destruction in Mormon-Ether gained the book of Moroni, a collection of minor texts associated with Mormon.
The boundaries between the first two groups are marked by Mormon’s editorial interruptions in the Words of Mormon, while the boundary between the second and third groups consists of Mormon’s additions to 3 Nephi (28:17-30:2).
I surmise that each of the three narrative cores grew or was structured sequentially, while the larger or smaller accretions were added at later times. Think of writing a journal: The bulk of it is material you add from day to day, while your list of “Top 10 hamburgers I’ve eaten” can be added much later and doesn’t have the same implications of chronological sequence. Not all additions to a text come at the end, but it is easier for them to be added where there’s an existing seam or boundary in the text. Consequently, the Book of Mormon isn’t only linear, but also chronologically layered, with its text potentially reflecting multiple time periods.
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At least, that’s what I think I’m reading when I read the Book of Mormon. You may have noticed that I keep saying “guess,” “suspect,” “surmise,” and similar. There’s no good way to verify any of this. On the one hand, what I see in the Book of Mormon seems very similar to other texts I’ve worked with where I can directly observe how they have been supplemented over decades or centuries. On the other hand, I’m well aware that the kind of guesswork I’m engaged in here has a modest chance of being right. There have been cases where I’ve thought that a certain text felt like it was missing its first half, and then I later found an edition containing the whole thing, confirming my guess. But other times I’ve been sure a particular passage was an insertion followed by a resumptive repetition, only to find the exact same wording in every version going back to the original.
III. Mormon’s sources
IIIa. Nephite literacy
IIIb. The material culture of Nephite literacy
IIIb note 1. A note on the uniformity of the Golden Plates
IIIc. The source structure of the Book of Mormon
IV. The puzzle of 3 Nephi
V. The permissibility and utility of philology for studying the Book of Mormon
Va. The permissibility of philology
Vb. The utility of philology
Vb1. Useful cautions
Vb2. What did the Nephites know about Nephi?
Vb3. The overdetermination of Nephite origins
Vb4. Jacob and Sherem
 Attributing all editorial work solely to Mormon is for example the approach in Thomas W. Mackay, “Mormon as Editor: A Study in Colophons, Headers, and Source Indicators,” Journal of Book of Mormon Studies 2.2 (1993): 90-109.
 Garly L. Sturgess, “The Book of Mosiah: Thoughts about Its Structure, Purposes, Themes, and Authorship,” Journal of Book of Mormon Studies 4.2 (1995): 108-10.
 Brant A. Gardner, “Two Authors: Two Approaches in the Book of Mormon,” Journal of Book of Mormon Studies 24.1 (2015): 258; Gardner, Labor Diligently, 164-73.