Mosiah 28:16 - 28:20

Mosiah 28:16 And whosoever has these things is called seer, after the manner of old times.

And anyone who had possession of the stones and could use them was called a seer. This was as in ancient time the same process. A seer is called and given these stones and he would then translate records which were of interest of the people and which the Lord wanted them to have.

Mosiah 28:17 Now after Mosiah had finished translating these records, behold, it gave an account of the people who were destroyed, from the time that they were destroyed back to the building of the great tower, at the time the Lord confounded the language of the people and they were scattered abroad upon the face of all the earth, yea, and even from that time back until the creation of Adam.

The record of the gold plates consisted of a people who originated from the tower of Babel and whose language was confounded and scattered all over the face of the earth. The record went back to Adam and Eve.

Mosiah 28:18 Now this account did cause the people of Mosiah to mourn exceedingly, yea, they were filled with sorrow; nevertheless it gave them much knowledge, in the which they did rejoice.

The translation of the gold plates gave them the knowledge they were seeking, however a reaction of sadness came over Mosiah's people as they read the chronicle of the history and destruction of a whole civilization.

Mosiah 28:19 And this account shall be written hereafter; for behold, it is expedient that all people should know the things which are written in this account.

The account of this civilization spoken of will be given at a later point in this abridged version of the history of the Nephites.
[It is interesting that the Book of Ether is in the book because of the efforts of Moroni and not Mormon. It could be that Mormon was killed before he could accomplish this. So when Moroni read in the abridgement by his father of Mosiah’s reign that it was supposed to be in the final edition, he went ahead and did it himself.
Or maybe Mormon knew that circumstances would not allow him to do this abridging and asked his son to do it.
Or the Book of Ether is a further condensing down of the original text by Mosiah. There is a lot of editorializing in the Book of Ether by Moroni so the original record, it seems to me was probably a lot longer then what we have in the Book of Ether. Mormon maybe was just going to put in the entire text as Mosiah translated it without comment or brief ones at best as he had done up to this point, but Moroni thought it was important to do more.]

Mosiah 28:20 And now, as I said unto you, that after king Mosiah had done these things, he took the plates of brass, and all the things which he had kept, and conferred them upon Alma, who was the son of Alma; yea, all the records, and also the interpreters, and conferred them upon him, and commanded him that he should keep and preserve them, and also keep a record of the people, handing them down from one generation to another, even as they had been handed down from the time that Lehi left Jerusalem.

After Mosiah had translated the gold plates found by Limhi’s scouts, he conferred all the records mentioned before hand into the hand of Alma Jr. Alma was further commanded to continue keeping a record of the people so that his record and the ones given him would continue down the generations. The interpreters were also handed over to Alma.