The Book of Mormon is a sacred record of peoples in ancient America and was engraved upon metal plates.
The Book of Mormon comprises fifteen main parts or divisions, known, with one exception, as books, usually designated by the name of their principal author. The first portion (the first six books, ending with Omni) is a translation from the small plates of Nephi. Between the books of Omni and Mosiah is an insert called the Words of Mormon. This insert connects the record engraved on the small plates with Mormon’s abridgment of the large plates.
The longest portion, from Mosiah through Mormon chapter 7, is a translation of Mormon’s abridgment of the large plates of Nephi. The concluding portion, from Mormon chapter 8 to the end of the volume, was engraved by Mormon’s son Moroni, who, after finishing the record of his father’s life, made an abridgment of the Jaredite record (as the book of Ether) and later added the parts known as the book of Moroni.
In or about the year A.D. 421, Moroni, the last of the Nephite prophet-historians, sealed the sacred record and hid it up unto the Lord, to be brought forth in the latter days, as predicted by the voice of God through His ancient prophets. In A.D. 1823, this same Moroni, then a resurrected personage, visited the Prophet Joseph Smith and subsequently delivered the engraved plates to him.
Read the Brief Explanation about the Book of Mormon: https://site.churchofjesuschrist.org/study/scriptures/bofm/explanation