Mormon had enough records that the presence of the small plates of Nephi was not obvious. He found them only after searching. Thus, he apparently collected large numbers of plates in multiple volumes. Probably each named book was a separate unit, among which the small plates of Nephi were not conspicuous in form.
The materials to which Mormon had access probably looked something like this:
• Separate bound volumes for the books of Lehi, Mosiah, Alma, Helaman, Nephi, and Nephi son of Nephi.
• Separate bound volumes for the record of Zeniff and, probably, for Ammon and the other missionary brothers where the source is not clearly stated.
• A separate bound volume for the record of Nephi3.
The book of Nephi3 is a special case because Mormon tells us that it parallels but is shorter than the version on the official plates. Therefore, individual prophets could and did make records. Since Ammon and Aaron almost certainly did, it seems likely that many shorter accounts from different people also existed, thus increasing Mormon’s source material. It seems improbable that all of these short, individual records were on gold plates since their technology apparently required government resources. (See commentary accompanying Omni 1:30, and 1 Nephi, Part 1: Context, Chapter 5, “Sources of the Sources.”) Nevertheless, he probably did not take all of these records at this time, but only the most recent set, upon which he added his record as did others who wrote on the plates of Nephi. He returns to this hill for records later, clearly indicating that there were more than those he took this first time (Morm. 4:23).
After retrieving the plates, Mormon set about his task of chronicling contemporary events on the plates of Nephi. That record is not, however, our current book of Mormon. He clearly states: “And upon the plates of Nephi I did make a full account of all the wickedness and abominations; but upon these plates I did forbear to make a full account of their wickedness and abominations” (v. 18).
Mormon contrasts the “full account” on “the plates of Nephi” with the abbreviated account on “these plates.” “These plates” are his abridgment, the record we know as the Book of Mormon. Mormon therefore wrote two histories, a first, full account for his contemporary audience and a later, abridged account for a future audience.