The Book of Mormon came to us in its current form through the work of a lot of different people both ancient and modern. Much of the work anciently was done by a prophet named Mormon, hence the title that we use for the book – The Book of Mormon: Another Testament of Jesus Christ. He served as abridger and editor of sacred and secular records that had been kept by prophets and other individuals over the years. As inspired, he selected passages that told of the history of some peoples in the Americas but more importantly, taught the gospel of Jesus Christ. He did all this while watching the end of his civilization – most of his people had either been killed in wars or had turned from following the statutes of God and joined with their enemies. Mormon would eventually be killed in battle, leaving his son Moroni to have charge over the sacred records and to finish the Book of Mormon. Moroni did this, burying the book in a stone box in the ground of what would later be upstate New York.
The plates Moroni buried in the ground were made of gold, which does not corrode or corrupt. Words were engraven onto the plates with great difficulty (see Jacob 4:1) so words were chosen judiciously not just because of the difficulty of engraving but also because of limited space; more importantly, words were chosen so as to help those who read the book draw nearer to Christ.
What we have as the current start of the Book of Mormon was not, in fact, the start of the book. There originally were 116 additional manuscript pages that were abridged by Mormon but these translated pages were lost when Martin Harris, who was helping Joseph Smith with the translation of the Book of Mormon, had convinced Joseph to let him take the manuscript home (so that he could show his wife, who was upset at the work Martin was doing with Joseph). These lost pages likely would have had some sort of introductory commentary by Mormon as well as a more secular history of the people of Nephi:
And now, I speak somewhat concerning that which I have written; for after I had made an abridgment from the plates of Nephi, down to the reign of this king Benjamin, of whom Amaleki spake, I searched among the records which had been delivered into my hands, and I found these plates, which contained this small account of the prophets, from Jacob down to the reign of this king Benjamin, and also many of the words of Nephi. (Words of Mormon 1:3).
Mormon made all his abridgments and then discovered the “small plates of Nephi” that included some history but were mainly focused on sacred things (particularly on Jesus Christ).
And now I, Nephi, do not give the genealogy of my fathers in this part of my record; neither at any time shall I give it after upon these plates which I am writing; for it is given in the record which has been kept by my father; wherefore, I do not write it in this work. For it sufficeth me to say that we are descendants of Joseph. And it mattereth not to me that I am particular to give a full account of all the things of my father, for they cannot be written upon these plates, for I desire the room that I may write of the things of God. For the fulness of mine intent is that I may persuade men to come unto the God of Abraham, and the God of Isaac, and the God of Jacob, and be saved. Wherefore, the things which are pleasing unto the world I do not write, but the things which are pleasing unto God and unto those who are not of the world. Wherefore, I shall give commandment unto my seed, that they shall not occupy these plates with things which are not of worth unto the children of men. (1 Nephi 6).
Moroni wrote his abridgments of the scriptural and historical records on gold plates bound together into a single unit; to the scriptures he edited, he added the unabridged small plates of Nephi. We do not know if the physical dimensions of the small plates of Nephi were the same as the plates Mormon made – it is possible but I’d be surprised if all the plates were the same size. Nephi’s plates were small though not necessarily because of their physical dimensions, they were small because there were not many of them and there was not room to write anything but the most important things of the Lord (the fact that so much of the words of Isaiah were included tells you something of the worth of the words of Isaiah – priceless).
As we put all this together, we see that Nephi thus wrote most of the first part of the Book of Mormon. As I just wrote, Nephi quotes extensively from Isaiah so many of his words are included. Nephi’s younger brother Jacob is the next author. In addition to what he writes, he quotes extensively from a prophet named Zenos. Jacob then passes the plates on to his son Enos, who writes a few words. His son Jarom next has the plates; he writes a little less than his father (there is not much room left on the plates and not much to add to what was previously written). Jarom gives the plates to his son Omni who writes what become three verses. Omni passes the plates to his son Amaron who then, after writing a little, gives them to his brother Chemish. Chemish gives them to his son Abinadom who then gives them to his son Amaleki. Amaleki fills up the rest of the small plates and then gives them to Benjamin, a righteous king and prophet.
So the lineage of the small plates of Nephi is thus [(b) = brother; (s) = son]: Nephi –> Jacob (b) –> Enos (s) –> Jarom (s) –> Omni (s) –> Amaron (s) –> Chemish (b) –> Abinadom (s) –> Amaleki (s) –> Benjamin (relationship unclear). That is seven, maybe eight generations right there – many hundreds of years. Each of those men wrote a portion of the Book of Mormon, albeit to varying degrees. That timeframe from Nephi to Benjamin was also covered by the other records Mormon abridged, which translation became lost.
That something is missing is obvious when we get to the Words of Mormon and read: “And now I, Mormon, being about to deliver up the record which I have been making into the hands of my son Moroni, behold I have witnessed almost all the destruction of my people, the Nephites.” (Words of Mormon 1:1). That’s an abrupt change of topic, something that sounds like it is act 2 of a play. From this we can assume that Mormon spent some time introducing the Book of Mormon and his work earlier (in the now lost portion of the book). He also likely had added his commentary throughout that portion of the plates, as he does throughout the rest of the Book of Mormon.
The rest of the Book of Mormon was mainly written by Mormon (who lived around 300-400 AD) with a few words by his son Moroni (who buried the gold plates around the year 421 AD). Mormon quotes extensively from various prophets and later, quotes directly from the Savior when He visits the Americas, but Mormon’s quoting and commentary are interwoven, often with little differentiation between commentary and quote. The book of Ether, which is near the end of the Book of Mormon is a transcription of a translation (done by Mosiah, father of king Benjamin) of records of a group of people called the Jaredites. They lived long before the Nephites, traveling to the Americas thousands of years BC (likely crossing over into the Americas via a northern route, such as from what is now {north}east Asia). Their civilization lasted through many wars for over a thousand years until the last of them was discovered by another group of people who had left Jerusalem shortly after Lehi’s family did (the Mulekites). These people had struggled through wars and loss of cultural and spiritual heritage. When their people was discovered, they were taught the language of the Nephites and joined together (both groups were of the house of Israel). Through this joining, the record of the Jaredites entered the possession of the Nephites:
And it came to pass that the people of Zarahemla, and of Mosiah, did unite together; and Mosiah was appointed to be their king. And it came to pass in the days of Mosiah, there was a large stone brought unto him with engravings on it; and he did interpret the engravings by the gift and power of God. And they gave an account of one Coriantumr, and the slain of his people. And Coriantumr was discovered by the people of Zarahemla; and he dwelt with them for the space of nine moons. It also spake a few words concerning his fathers. And his first parents came out from the tower, at the time the Lord confounded the language of the people; and the severity of the Lord fell upon them according to his judgments, which are just; and their bones lay scattered in the land northward. (Omni 1:19-22).
There, now we’ve covered most of those who wrote what is now the Book of Mormon (Mormon, Nephi, Jacob, and Moroni are the four largest contributors). Because Mormon did most of the organizational and editorial work, the book is named in honor of him. He compiled a remarkable book that was not written for his people – they were almost all dead – it was written for the purpose of coming forth in these days to teach people of Jesus Christ and to add to the witness of the Bible of the truthfulness of Christ’s mission.