The text here seems to be referring to the other books as going “from the Gentiles unto them”— that is, to the descendants of Laman and Lemuel, “the remnant of the seed of my brethren”—and “also to the Jews”. Unlike the Bible, these other scriptures would come from the Gentiles themselves and would go to both the Lamanites and the Jews. In other words, the prepositional phrase “to the Jews” (and its following relative clause “which were scattered upon all the face of the earth”) is conjoined with the preceding “unto them”.
Obviously, this prepositional phrase along with its attached relative clause interrupts the long gerundive clause “unto the convincing of the Gentiles and the remnant of the seed of my brethren … that the records of the prophets and of the twelve apostles of the Lamb are true”. Joseph Smith’s solution to this difficulty was to include the Jews as among those who would be convinced by these additional scriptures, which is not what the original text actually says. In his editing for the 1837 edition, Joseph Smith simply deleted the preposition to and thus altered the meaning.
There is nothing inappropriate about conjoining the prepositions unto and to within a conjoined construction, as in the following examples scattered throughout the Book of Mormon:
Note that several of these also have “and also to”, just like the text originally in 1 Nephi 13:39.
Given that the emphasis in the extended passage is on scriptural books going from one people to another, the preposition to is wholly appropriate in 1 Nephi 13:39 and should therefore be restored, even though it has a more complicated reading.
Summary: Restore the preposition to in the earliest text for 1 Nephi 13:39; this passage refers to other scriptures being delivered to the Jews (“and also to the Jews”); although this construction is complex, it does make sense.