There is a presumption that "the Book of Mormon" means the book "Book of Mormon" we have now. That is, the one you can get off the shelf at Deseret Book. That is not the only possible meaning of the words.
Nephi records he made two sets of plates. On one he recorded the "full record" of his people. We do not have that record. On the other he included little history and a summary of his religious teachings and prophecies. (1 Ne. 9: 2) The small plates we have are devoted primarily to his "ministry." (1 Ne. 9: 4.) When Nephi prepared the first, larger plates, he was unaware he would later receive a commandment to make the second, shorter record devoted to only his ministry. (1 Ne. 19: 1-3.)
The commandment to make the second set of plates was not given until after Lehi died in the promised land, and Nephi and those who followed him separated from his older brothers Laman and Lemuel. (2 Ne. 5: 30.) This would have been several decades after the events in and around Jerusalem.
Mormon did not use Nephi's "small plates" to abridge in his original book. He used Nephi's large plates, containing "more history part" of the people. But, after finishing his abridgement, he attached the small plates to his abridged record, noting that the small plates he attached contained "this small account" of the prophets from Jacob down to King Benjamin. (Words of Mormon 1: 3.) Within the small plates Mormon explained there were "many of the words of Nephi" (Id.). For all of Nephi's words, we would need access to the large plates.
The "Book of Mormon" included: 1) Mormon's summary of the Nephite records, which was based on the larger plates and not the smaller ones, 2) The small plates of Nephi, 3) Moroni's translation of part of the Jaredite records, 4) Some correspondence between Mormon and Moroni, along with Moroni's final warnings, and 5) An extensive, sealed and untranslated record containing information not yet revealed to us (2 Ne. 27: 6-8). We no longer have a portion of part 1, it having been lost through Martin Harris' neglect. It was not re-translated after the first version was lost. (See D&C 10: 30.) Part 5, or the sealed portion of the record, contains a revelation from God of everything from the beginning to the end (2 Ne. 27: 7.)
When "the Book of Mormon" is said to contain the "fullness of the Gospel" (D&C 42: 12) is that referring to what we have now (parts 2, 3, 4)? Does it or did it also include what was originally included by Mormon, but has been lost to us as a result of Martin Harris (part 1)? Does it include the sealed portion of the record we have never been given (part 5)? Although the traditional discussion presumes what we now have (parts 2, 3, 4) are what is meant by "the Book of Mormon" there are other possible meanings.