Mormon openly admits that his record is an abridgement (WofM 1:3). The number of records he uses to make his abridgement is undisclosed but is probably very numerous. …there are many records kept…which are particular and very large…behold, there are many books and many records of every kind, and they have been kept chiefly by the Nephites.’ (Hel 3:13-15)
Certainly, the Book of Mormon is an abbreviated history, designed to bring us to Christ not to inform us of all the doings of the Nephites and Lamanites. The above scripture makes it clear that many other records were kept. Some of these other records were seen by the prophet Joseph Smith and Oliver Cowdery while in a cave in the hill Cumorah. Brigham Young tells the story:
“Oliver Cowdery went with the Prophet Joseph when he deposited these plates… When Joseph got the plates, the angel instructed him to carry them back to the hill Cumorah, which he did. Oliver says that when Joseph and Oliver went there, the hill opened, and they walked into a cave, in which there was a large and spacious room. He says he did not think, at the time, whether they had the light of the sun or artificial light; but that it was just as light as day. They laid the plates on a table; it was a large table that stood in the room. Under this table there was a pile of plates as much as two feet high, and there were altogether in this room more plates than probably many wagon loads; they were piled up in the corners and along the walls. The first time they went there the sword of Laban hung upon the wall; but when they went again it had been taken down and laid upon the table across the gold plates; it was unsheathed, and on it was written these words: ’This sword will never be sheathed again until the kingdoms of this world become the kingdom of our God and his Christ.’” (Journal of Discourses, vol. 19, p. 40)