In any and all ages the Lord's people have had scripture of their own writing. Whenever true religion existed among the Jews, they had prophets who added to the then extant scriptural records.
The Nephites kept their own scriptural records, as have all of the scattered branches of the house of Israel which were led away by the Lord. It appears that other families, like that of Lehi and Ishmael, were led from time to time by the hand of the Lord to various places throughout the earth.They would have kept scriptural records which will someday be restored to us. In this passage, the Lord announces a future day when he will speak unto all nations of the earth and they also will write it. Of a surety, as long as there are righteous people upon the earth, new scriptural records will be written.