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Who are Zenos, Zenock and Neum?
Outside of what is written in the Book of Mormon, we have no record of…these ancient prophets….Their writings must have been recorded on the [brass] plates.
Joseph Fielding Smith, Answers to Gospel Questions, 4:138

First Nephi chapters 19 through 22 introduce us to Zenos, Zenock, and Neum and give us our first quotations from Isaiah. Little is known of the prophets Zenos, Zenock, and Neum, although their importance to the Book of Mormon prophets is evident in that Nephi, Jacob, Alma, Amulek, Nephi the son of Helaman, Samuel, and Mormon all quote from them. They are important to the Nephites for at least three reasons:
First, the Nephites appear to be descendants of these prophets and of Joseph who was sold into Egypt (3 Nephi 10:16-17).
Second, these prophets spoke of that which would happen to all the house of Israel as well as to Lehi's seed (1 Nephi 19:16-17; Helaman 15:11).
And third, and probably most important, they testified of Christ, and hundreds of years before his birth prophesied in detail of his atonement and the circumstances surrounding it (1 Nephi 19:10-12; Alma 33:12-17). We also learn that these messianic prophets gave their lives for their testimonies of Christ (Alma 33:17; Helaman 8:19).
Exactly when and where Zenos, Zenock, and Neum lived is not known except we do know that they lived prior to Lehi's departure from Jerusalem, otherwise their writings would not have appeared on the brass plates. It is likely that they lived after the time of Abraham and before the days of Isaiah, and that Zenos preceded Zenock. This assumption is based on the following scripture, and assumes that since Isaiah and Jeremiah are listed in chronological order, the other prophets are also.

And now I would that ye should know, that even since the days of Abraham there have been many prophets that have testified these things; yea, behold, the prophet Zenos did testify boldly; for the which he was slain.
And behold, also Zenock, and also Ezias, and also Isaiah, and Jeremiah, (Jeremiah being that same prophet who testified of the destruction of Jerusalem) and now we know that Jerusalem was destroyed according to the words of Jeremiah. O then why not the Son of God come, according to his prophecy? (Helaman 8:19-20.)

Since these prophets were descended from Joseph, it is postulated by some that they may have lived in the north of Israel, rather than in Judah and that is the reason their prophecies were not recorded in Jewish scripture. It may also be possible that these plain and precious prophecies were deleted by unrighteous Jews who did not want these statements on the death of Christ in their scriptures (see 1 Nephi 19:13-14). Until the Lord reveals more information, however, we won't know the whole story.
Monte S. Nyman and Charles D. Tate, Jr., eds., First Nephi: The Doctrinal Foundation, p.272-273

Zenos

I do not think I overstate the matter when I say that next to Isaiah himself—who is the prototype, pattern, and model for all the prophets—there was not a greater prophet in all Israel than Zenos. And our knowledge of his inspired writings is limited to the quotations and paraphrasing summaries found in the Book of Mormon.
Bruce R. McConkie, The Doctrinal Restoration, Chap. 1 in The Joseph Translation, p. 17